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An invasion is a
military offensive An offensive is a military operation that seeks through an aggressive projection of armed forces to occupy territory, gain an objective or achieve some larger strategic, operational, or tactical goal. Another term for an offensive often used b ...
in which large numbers of
combatants Combatant is the legal status of an individual who has the right to engage in hostilities during an armed conflict. The legal definition of "combatant" is found at article 43(2) of Additional Protocol I (AP1) to the Geneva Conventions of 1949. ...
of one
geopolitical Geopolitics (from Greek γῆ ''gê'' "earth, land" and πολιτική ''politikḗ'' "politics") is the study of the effects of Earth's geography (human and physical) on politics and international relations. While geopolitics usually refers to ...
entity An entity is something that exists as itself, as a subject or as an object, actually or potentially, concretely or abstractly, physically or not. It need not be of material existence. In particular, abstractions and legal fictions are usually ...
aggressively enter
territory A territory is an area of land, sea, or space, particularly belonging or connected to a country, person, or animal. In international politics, a territory is usually either the total area from which a state may extract power resources or a ...
owned by another such entity, generally with the objective of either: conquering; liberating or re-establishing control or authority over a territory; forcing the partition of a country; altering the established
government A government is the system or group of people governing an organized community, generally a state. In the case of its broad associative definition, government normally consists of legislature, executive, and judiciary. Government is ...
or gaining concessions from said government; or a combination thereof. An invasion can be the cause of a
war War is an intense armed conflict between states, governments, societies, or paramilitary groups such as mercenaries, insurgents, and militias. It is generally characterized by extreme violence, destruction, and mortality, using regular o ...
, be a part of a larger strategy to end a war, or it can constitute an entire war in itself. Due to the large scale of the operations associated with invasions, they are usually strategic in planning and execution.


History

Archaeological evidence indicates that invasions have been frequent occurrences since
prehistory Prehistory, also known as pre-literary history, is the period of human history between the use of the first stone tools by hominins 3.3 million years ago and the beginning of recorded history with the invention of writing systems. The use ...
. In antiquity, before
radio Radio is the technology of signaling and communicating using radio waves. Radio waves are electromagnetic waves of frequency between 30 hertz (Hz) and 300 gigahertz (GHz). They are generated by an electronic device called a transmi ...
communication Communication (from la, communicare, meaning "to share" or "to be in relation with") is usually defined as the transmission of information. The term may also refer to the message communicated through such transmissions or the field of inqui ...
s and fast
transportation Transport (in British English), or transportation (in American English), is the intentional movement of humans, animals, and goods from one location to another. Modes of transport include air, land (rail and road), water, cable, pipeline, ...
, the only way for a military to ensure adequate reinforcements was to move armies as one massive force. This, by its very nature, led to the strategy of invasion. With invasion came cultural exchanges in government,
religion Religion is usually defined as a social- cultural system of designated behaviors and practices, morals, beliefs, worldviews, texts, sanctified places, prophecies, ethics, or organizations, that generally relates humanity to supernatural, ...
, philosophy, and
technology Technology is the application of knowledge to reach practical goals in a specifiable and Reproducibility, reproducible way. The word ''technology'' may also mean the product of such an endeavor. The use of technology is widely prevalent in me ...
that shaped the development of much of the
ancient world Ancient history is a time period from the beginning of writing and recorded human history to as far as late antiquity. The span of recorded history is roughly 5,000 years, beginning with the Sumerian cuneiform script. Ancient history cov ...
. Before the days of package tours and cut-price airlines, military invasions functioned as a standard major form of proto-
tourism Tourism is travel for pleasure or business; also the theory and practice of touring, the business of attracting, accommodating, and entertaining tourists, and the business of operating tours. The World Tourism Organization defines tourism mor ...
– bringing large numbers of foreign visitors into new environments, with the consequential social, cultural and economic impacts on indigenous populations and on the invaders.


Defenses

States with potentially hostile neighbors typically adopt defensive measures to delay or forestall an invasion. In addition to utilizing geographical barriers such as
river A river is a natural flowing watercourse, usually freshwater, flowing towards an ocean, sea, lake or another river. In some cases, a river flows into the ground and becomes dry at the end of its course without reaching another body of w ...
s,
marsh A marsh is a wetland that is dominated by herbaceous rather than woody plant species.Keddy, P.A. 2010. Wetland Ecology: Principles and Conservation (2nd edition). Cambridge University Press, Cambridge, UK. 497 p Marshes can often be found a ...
es, or rugged
terrain Terrain or relief (also topographical relief) involves the vertical and horizontal dimensions of land surface. The term bathymetry is used to describe underwater relief, while hypsometry studies terrain relative to sea level. The Latin wo ...
, these measures have historically included
fortification A fortification is a military construction or building designed for the defense of territories in warfare, and is also used to establish rule in a region during peacetime. The term is derived from Latin ''fortis'' ("strong") and ''facere' ...
s. Such a defense can be intended to actively prevent invading forces from entering the country by means of an extended and well-defended barrier; the
Great Wall of China The Great Wall of China (, literally "ten thousand ''li'' wall") is a series of fortifications that were built across the historical northern borders of ancient Chinese states and Imperial China as protection against various nomadic gro ...
, Hadrian's Wall, and the
Danewerk The Danevirke or Danework (modern Danish spelling: ''Dannevirke''; in Old Norse; ''Danavirki'', in German; ''Danewerk'', literally meaning '' earthwork of the Danes'') is a system of Danish fortifications in Schleswig-Holstein, Germany. This his ...
are famous examples. Such barriers have also included
trench A trench is a type of excavation or in the ground that is generally deeper than it is wide (as opposed to a wider gully, or ditch), and narrow compared with its length (as opposed to a simple hole or pit). In geology, trenches result from ero ...
lines and, in more modern times, minefields,
cameras A camera is an optical instrument that can capture an image. Most cameras can capture 2D images, with some more advanced models being able to capture 3D images. At a basic level, most cameras consist of sealed boxes (the camera body), with a ...
, and motion-sensitive sensors. However, these barriers can require a large military force to provide the defense, as well as maintain the equipment and positions, which can impose a great
economic An economy is an area of the production, distribution and trade, as well as consumption of goods and services. In general, it is defined as a social domain that emphasize the practices, discourses, and material expressions associated with the ...
burden on the country. Some of those same techniques can also be turned against defenders, used to keep them from escape or resupply. During
Operation Starvation Operation Starvation was a naval mining operation conducted in World War II by the United States Army Air Forces to disrupt Japanese shipping. Operation The mission was initiated at the insistence of Admiral Chester Nimitz who wanted his naval ...
, Allied forces used airdropped mines to severely disrupt
Japanese Japanese may refer to: * Something from or related to Japan, an island country in East Asia * Japanese language, spoken mainly in Japan * Japanese people, the ethnic group that identifies with Japan through ancestry or culture ** Japanese diaspor ...
logistical operations within their own borders. Alternatively, the fortifications can be built up at a series of sites, such as castles or forts placed near a border. These structures are designed to delay an invasion long enough for the defending nation to mobilize an army of a size sufficient for defense or, in some cases, counter-invasion—such as, for example, the Maginot Line. Forts can be positioned so that the garrisons can interdict the
supply line Supply may refer to: *The amount of a resource that is available **Supply (economics), the amount of a product which is available to customers **Materiel, the goods and equipment for a military unit to fulfill its mission *Supply, as in confidenc ...
s of the invaders. The theory behind these spaced forts is that the invader cannot afford to bypass these defenses, and so must lay
siege A siege is a military blockade of a city, or fortress, with the intent of conquering by attrition, or a well-prepared assault. This derives from la, sedere, lit=to sit. Siege warfare is a form of constant, low-intensity conflict characteriz ...
to the structures. In modern times, the notion of constructing large-scale static defenses to combat land-based threats has largely become obsolete. The use of precision air campaigns and large-scale
mechanization Mechanization is the process of changing from working largely or exclusively by hand or with animals to doing that work with machinery. In an early engineering text a machine is defined as follows: In some fields, mechanization includes the ...
have made lighter, more mobile defenses desirable to military planners. The obsolescence of large fortifications was displayed by the failure of the Maginot Line in the beginning of World War Two. Nations defending against modern invasions normally use large population centers such as
cities A city is a human settlement of notable size.Goodall, B. (1987) ''The Penguin Dictionary of Human Geography''. London: Penguin.Kuper, A. and Kuper, J., eds (1996) ''The Social Science Encyclopedia''. 2nd edition. London: Routledge. It can be def ...
or
town A town is a human settlement. Towns are generally larger than villages and smaller than cities, though the criteria to distinguish between them vary considerably in different parts of the world. Origin and use The word "town" shares an ori ...
s as defensive points. The invader must capture these points to destroy the defender's ability to wage war. The defender uses mobile
armored Armour (British English) or armor (American English; see spelling differences) is a covering used to protect an object, individual, or vehicle from physical injury or damage, especially direct contact weapons or projectiles during combat, or f ...
and
infantry Infantry is a military specialization which engages in ground combat on foot. Infantry generally consists of light infantry, mountain infantry, motorized infantry & mechanized infantry, airborne infantry, air assault infantry, and mar ...
divisions to protect these points, but the defenders are still very mobile and can normally retreat. A prominent example of the use of cities as fortifications can be seen in the Iraqi Army's stands in the
2003 invasion of Iraq The 2003 invasion of Iraq was a United States-led invasion of the Republic of Iraq and the first stage of the Iraq War. The invasion phase began on 19 March 2003 (air) and 20 March 2003 (ground) and lasted just over one month, including 26 ...
at
Baghdad Baghdad (; ar, بَغْدَاد , ) is the capital of Iraq and the second-largest city in the Arab world after Cairo. It is located on the Tigris near the ruins of the ancient city of Babylon and the Sassanid Persian capital of Ctesiphon. I ...
,
Tikrit Tikrit ( ar, تِكْرِيت ''Tikrīt'' , Syriac: ܬܲܓܪܝܼܬܼ ''Tagrīṯ'') is a city in Iraq, located northwest of Baghdad and southeast of Mosul on the Tigris River. It is the administrative center of the Saladin Governorate. , it h ...
, and
Basra Basra ( ar, ٱلْبَصْرَة, al-Baṣrah) is an Iraqi city located on the Shatt al-Arab. It had an estimated population of 1.4 million in 2018. Basra is also Iraq's main port, although it does not have deep water access, which is han ...
in the major combat in the
Iraq War {{Infobox military conflict , conflict = Iraq War {{Nobold, {{lang, ar, حرب العراق (Arabic) {{Nobold, {{lang, ku, شەڕی عێراق ( Kurdish) , partof = the Iraq conflict and the War on terror , image ...
. A defender can also use these mobile assets to precipitate a counteroffensive like the
Soviet The Soviet Union,. officially the Union of Soviet Socialist Republics. (USSR),. was a transcontinental country that spanned much of Eurasia from 1922 to 1991. A flagship communist state, it was nominally a federal union of fifteen nation ...
Red Army The Workers' and Peasants' Red Army ( Russian: Рабо́че-крестья́нская Кра́сная армия),) often shortened to the Red Army, was the army and air force of the Russian Soviet Federative Socialist Republic and, afte ...
at the
Battle of Kursk The Battle of Kursk was a major World War II Eastern Front engagement between the forces of Nazi Germany and the Soviet Union near Kursk in the southwestern USSR during late summer 1943; it ultimately became the largest tank battle in history ...
or the
Northern Alliance The Northern Alliance, officially known as the United Islamic National Front for the Salvation of Afghanistan ( prs, جبهه متحد اسلامی ملی برای نجات افغانستان ''Jabha-yi Muttahid-i Islāmi-yi Millī barāyi Nijāt ...
in
Afghanistan Afghanistan, officially the Islamic Emirate of Afghanistan,; prs, امارت اسلامی افغانستان is a landlocked country located at the crossroads of Central Asia and South Asia. Referred to as the Heart of Asia, it is bordere ...
. However, static emplacements remain useful in both defense against naval attacks and defense against air attacks.
Naval mine A naval mine is a self-contained explosive device placed in water to damage or destroy surface ships or submarines. Unlike depth charges, mines are deposited and left to wait until they are triggered by the approach of, or contact with, any ...
s are still an inexpensive but effective way to defend
port A port is a maritime facility comprising one or more wharves or loading areas, where ships load and discharge cargo and passengers. Although usually situated on a sea coast or estuary, ports can also be found far inland, such as H ...
s and choke off supply lines. Large static air defense systems that combine antiaircraft guns with
missile launchers A rocket launcher is a weapon that launches an unguided, rocket-propelled projectile. History The earliest rocket launchers documented in imperial China consisted of arrows modified by the attachment of a rocket motor to the shaft a few in ...
are still the best way to defend against air attacks. Such systems were used effectively by the
North Vietnam North Vietnam, officially the Democratic Republic of Vietnam (DRV; vi, Việt Nam Dân chủ Cộng hòa), was a socialist state supported by the Soviet Union (USSR) and the People's Republic of China (PRC) in Southeast Asia that existed f ...
ese around
Hanoi Hanoi or Ha Noi ( or ; vi, Hà Nội ) is the capital and second-largest city of Vietnam. It covers an area of . It consists of 12 urban districts, one district-leveled town and 17 rural districts. Located within the Red River Delta, Hanoi is ...
. Also, the
United States The United States of America (U.S.A. or USA), commonly known as the United States (U.S. or US) or America, is a country primarily located in North America. It consists of 50 states, a federal district, five major unincorporated territori ...
has invested considerable time and money into the construction of a
National Missile Defense National missile defense (NMD) is a generic term for a type of missile defense intended to shield an entire country against incoming missiles, such as intercontinental ballistic missile (ICBMs) or other ballistic missiles. This is also used ...
system, a static defense grid intended to intercept nuclear intercontinental ballistic missiles. Island nations, such as the
United Kingdom The United Kingdom of Great Britain and Northern Ireland, commonly known as the United Kingdom (UK) or Britain, is a country in Europe, off the north-western coast of the European mainland, continental mainland. It comprises England, Scotlan ...
or Japan, and continental states with extensive
coast The coast, also known as the coastline or seashore, is defined as the area where land meets the ocean, or as a line that forms the boundary between the land and the coastline. The Earth has around of coastline. Coasts are important zones in ...
s, such as the United States, have utilized a significant naval presence to forestall an invasion of their country, rather than fortifying their border areas. A successful naval defense, however, usually requires a preponderance of naval power and the ability to sustain and service that defense force. In particularly large nations, the defending force may also retreat to facilitate a
counterattack A counterattack is a tactic employed in response to an attack, with the term originating in "war games". The general objective is to negate or thwart the advantage gained by the enemy during attack, while the specific objectives typically seek ...
by drawing the invaders deeper into hostile territory. One effect of this tactic is that the invading force becomes too spread out, making supply difficult and making the lines more susceptible to attack. This tactic, although costly, helped the Soviets stop the German advance at Stalingrad. It can also cause the invading force to extend too far, allowing a
pincer movement The pincer movement, or double envelopment, is a military maneuver in which forces simultaneously attack both flanks (sides) of an enemy formation. This classic maneuver holds an important foothold throughout the history of warfare. The pin ...
to cut them off from reinforcements. This was the cause of the British defeat at the
Battle of Cowpens The Battle of Cowpens was an engagement during the American Revolutionary War fought on January 17, 1781 near the town of Cowpens, South Carolina, between U.S. forces under Brigadier General Daniel Morgan and Kingdom of Great Britain, British for ...
during the
American Revolutionary War The American Revolutionary War (April 19, 1775 – September 3, 1783), also known as the Revolutionary War or American War of Independence, was a major war of the American Revolution. Widely considered as the war that secured the independence of t ...
. Finally, sending too many reinforcements can leave too few defenders in the attackers' territory, allowing a counter-invasion from other areas, as happened in the Second Punic War.


Methods

There are many different methods by which an invasion can take place, each method having arguments both in their favour and against. These include invasion by land, sea, or air, or any combination of these methods.


By land

Invasion over land is the straightforward entry of armed forces into an area using existing land connections, usually crossing
border Borders are usually defined as geographical boundaries, imposed either by features such as oceans and terrain, or by political entities such as governments, sovereign states, federated states, and other subnational entities. Political borders c ...
s or otherwise defined zones, such as a
demilitarized zone A demilitarized zone (DMZ or DZ) is an area in which treaties or agreements between nations, military powers or contending groups forbid military installations, activities, or personnel. A DZ often lies along an established frontier or bounda ...
, overwhelming defensive emplacements and structures. Although this tactic often results in a quick victory, troop movements are relatively slow and subject to disruption by terrain and weather. Furthermore, it is hard to conceal plans for this method of invasion, as most geopolitical entities take defensive positions in areas that are most vulnerable to the methods mentioned above. In modern warfare, invasion by land often takes place after, or sometimes during, attacks on the target by other means. Air strikes and cruise missiles launched from ships at sea are a common method of "softening" the target. Other, more subtle, preparations may involve secretly garnering popular support, assassinating potentially threatening political or military figures, and closing off supply lines where they cross into neighboring countries. In some cases, those other means of attack eliminate the need for ground assault; the 1945 atomic-bombing of Hiroshima and Nagasaki ultimately made it unnecessary for the Allies to invade the Japanese home islands with infantry troops. In cases such as this, while some ground troops are still needed to occupy the conquered territory, they are allowed to enter under the terms of a
treaty A treaty is a formal, legally binding written agreement between actors in international law. It is usually made by and between sovereign states, but can include international organizations, individuals, business entities, and other legal pe ...
and as such are no longer invaders. As unmanned, long-range combat evolves, the instances of basic overland invasion become fewer; often the conventional fighting is effectively over before the infantry arrives in the role of peacekeepers (see " Applications regarding non-state combatants" in this article).


By sea

Invasion by sea is the use of a body of water to facilitate the entry of armed forces into an area, often a landmass adjoining the body of water or an island. This is generally used either in conjunction with another method of invasion, and especially before the invention of flight, for cases in which there is no other method to enter the territory in question. Arguments in favor of this method usually consist of the ability to perform a surprise attack from sea, or that naval defenses of the area in question are inadequate to repel such an attack. However, the large amount of specialized equipment, such as
amphibious vehicle An amphibious vehicle (or simply amphibian), is a vehicle that is a means of transport viable on land as well as on or under water. Amphibious vehicles include amphibious Amphibious cycle, bicycles, Amphibious ATV, ATVs, Amphibious automobile, ca ...
s and the difficulty of establishing defenses—usually with a resulting high casualty count—in exchange for a relatively small gain, are often used as arguments against such an invasion method. Underwater hazards and a lack of good cover are very common problems during invasions from the sea. At the Battle of Tarawa, Marine landing craft became hung up on a
coral reef A coral reef is an underwater ecosystem characterized by reef-building corals. Reefs are formed of colonies of coral polyps held together by calcium carbonate. Most coral reefs are built from stony corals, whose polyps cluster in groups. C ...
and were shelled from the beach. Other landers were sunk before they could reach the shore, and the
tank A tank is an armoured fighting vehicle intended as a primary offensive weapon in front-line ground combat. Tank designs are a balance of heavy firepower, strong armour, and good battlefield mobility provided by tracks and a powerful engi ...
s they were carrying were stranded in the water. Most of the few survivors of the first wave ended up pinned down on the beach. The island was conquered but at a heavy cost, and the loss of life sparked mass protests from civilians in the United States.


By air

Invasion by air is an invention of the 20th century and
modern warfare Modern warfare is warfare that is in notable contrast with previous military concepts, methods, and technology, emphasizing how combatants must modernize to preserve their battle worthiness. As such, it is an evolving subject, seen different ...
. The idea involves sending military units into a territory by
aircraft An aircraft is a vehicle that is able to fly by gaining support from the air. It counters the force of gravity by using either static lift or by using the dynamic lift of an airfoil, or in a few cases the downward thrust from jet engine ...
. The aircraft either land, allowing the military units to debark and attempt their objective, or the troops exit the aircraft while still in the air, using parachutes or similar devices to land in the territory being invaded. Many times air assaults have been used to pave the way for a ground- or sea-based invasion, by taking key positions deep behind enemy lines such as bridges and crossroads, but an entirely air-based invasion has never succeeded. Two immediate problems are resupply and reinforcement. A large airborne force cannot be adequately supplied without meeting up with ground forces; an airborne force too small simply places themselves into an immediate envelopment situation. Arguments in favor of this method generally relate to the ability to target specific areas that may not necessarily be easily accessible by land or sea, a greater chance of surprising the enemy and overwhelming defensive structures, and, in many cases, the need for a reduced number of forces due to the element of surprise. Arguments against this method typically involve capacity to perform such an invasion—such as the sheer number of planes that would be needed to carry a sufficient number of troops—and the need for a high level of
intelligence Intelligence has been defined in many ways: the capacity for abstraction, logic, understanding, self-awareness, learning, emotional knowledge, reasoning, planning, creativity, critical thinking, and problem-solving. More generally, it can be des ...
in order for the invasion to be successful. The closest examples to a true air invasion are the
Battle of Crete The Battle of Crete (german: Luftlandeschlacht um Kreta, el, Μάχη της Κρήτης), codenamed Operation Mercury (german: Unternehmen Merkur), was a major Axis airborne and amphibious operation during World War II to capture the island ...
,
Operation Thursday The Chindits, officially as Long Range Penetration Groups, were special operations units of the British and Indian armies which saw action in 1943–1944 during the Burma Campaign of World War II. The British Army Brigadier Orde Wingate form ...
(the
Chindits The Chindits, officially as Long Range Penetration Groups, were special operations units of the British and Indian armies which saw action in 1943–1944 during the Burma Campaign of World War II. The British Army Brigadier Orde Wingate form ...
second operation during the Burma Campaign) and Operation Market Garden. The latter was an assault on the German-occupied
Netherlands ) , anthem = ( en, "William of Nassau") , image_map = , map_caption = , subdivision_type = Sovereign state , subdivision_name = Kingdom of the Netherlands , established_title = Before independence , established_date = Spanish Netherl ...
conducted in September 1944. Nearly 35,000 men were dropped by parachute and
glider Glider may refer to: Aircraft and transport Aircraft * Glider (aircraft), heavier-than-air aircraft primarily intended for unpowered flight ** Glider (sailplane), a rigid-winged glider aircraft with an undercarriage, used in the sport of glidin ...
into enemy territory in an attempt to capture bridges from the Germans and make way for the Allies' advance. However, even with such a massive force taking the Germans completely by surprise, the assault was a tactical failure and after 9 days of fighting the Allies managed only to escape back to their own lines, having sustained over 18,000 casualties. In the 21st century, as vast improvements are made in anti-aircraft defenses, it seems that the air invasion is a strategy whose time may never come.


Pacification

Once political boundaries and military lines have been breached, pacification of the region is the final, and arguably the most important, goal of the invading force. After the defeat of the regular military, or when one is lacking, continued opposition to an invasion often comes from civilian or paramilitary
resistance movement A resistance movement is an organized effort by some portion of the civil population of a country to withstand the legally established government or an occupying power and to disrupt civil order and stability. It may seek to achieve its objective ...
s. Complete pacification of an occupied country can be difficult, and usually impossible, but popular support is vital to the success of any invasion. Media propaganda such as leaflets, books, and radio broadcasts can be used to encourage resistance fighters to surrender and to dissuade others from joining their cause. Pacification, often referred to as "the winning of hearts and minds", reduces the desire for civilians to take up resistance. This may be accomplished through reeducation, allowing conquered citizens to participate in their government, or, especially in impoverished or besieged areas, simply by providing food, water, and shelter. Sometimes displays of military might are used; invading forces may assemble and parade through the streets of conquered towns, attempting to demonstrate the futility of any further fighting. These displays may also include public
executions Capital punishment, also known as the death penalty, is the state-sanctioned practice of deliberately killing a person as a punishment for an actual or supposed crime, usually following an authorized, rule-governed process to conclude that t ...
of enemy soldiers, resistance fighters, and other conspirators. Particularly in antiquity, the death or imprisonment of a popular leader was sometimes enough to bring about a quick surrender. However, this has often had the unintended effect of creating
martyr A martyr (, ''mártys'', "witness", or , ''marturia'', stem , ''martyr-'') is someone who suffers persecution and death for advocating, renouncing, or refusing to renounce or advocate, a religious belief or other cause as demanded by an externa ...
s around which popular resistance can rally. An example of which was Sir
William Wallace Sir William Wallace ( gd, Uilleam Uallas, ; Norman French: ; 23 August 1305) was a Scottish knight who became one of the main leaders during the First War of Scottish Independence. Along with Andrew Moray, Wallace defeated an English army ...
, who, centuries after his execution by the English, is still a symbol of Scottish nationalism.


Support


Logistics

Without a steady flow of supplies, an invading force will soon find itself retreating. Before his invasion of
Greece Greece,, or , romanized: ', officially the Hellenic Republic, is a country in Southeast Europe. It is situated on the southern tip of the Balkans, and is located at the crossroads of Europe, Asia, and Africa. Greece shares land borders ...
,
Xerxes I Xerxes I ( peo, 𐎧𐏁𐎹𐎠𐎼𐏁𐎠 ; grc-gre, Ξέρξης ; – August 465 BC), commonly known as Xerxes the Great, was the fourth King of Kings of the Achaemenid Empire, ruling from 486 to 465 BC. He was the son and successor of D ...
spent three years amassing supplies from all over Asia;
Herodotus Herodotus ( ; grc, , }; BC) was an ancient Greek historian and geographer A geographer is a physical scientist, social scientist or humanist whose area of study is geography, the study of Earth's natural environment and human society ...
wrote that the
Persian Persian may refer to: * People and things from Iran, historically called ''Persia'' in the English language ** Persians, the majority ethnic group in Iran, not to be conflated with the Iranic peoples ** Persian language, an Iranian language of the ...
army was so large it "drank the rivers dry". In most invasions, even in modern times, many fresh supplies are gathered from the invaded territories themselves. Before the
laws of war The law of war is the component of international law that regulates the conditions for initiating war (''jus ad bellum'') and the conduct of warring parties (''jus in bello''). Laws of war define sovereignty and nationhood, states and territor ...
, invaders often relied heavily on the supplies they would win by conquering towns along the way. During the Second Punic War, for example, Hannibal diverted his army to conquer cities simply to gather supplies; his strategy in crossing the
Alps The Alps () ; german: Alpen ; it, Alpi ; rm, Alps ; sl, Alpe . are the highest and most extensive mountain range system that lies entirely in Europe, stretching approximately across seven Alpine countries (from west to east): France, Swi ...
necessitated traveling with as few provisions as possible, expecting the
Roman Roman or Romans most often refers to: *Rome, the capital city of Italy *Ancient Rome, Roman civilization from 8th century BC to 5th century AD *Roman people, the people of ancient Rome *'' Epistle to the Romans'', shortened to ''Romans'', a lette ...
stores to sustain them when they had breached the border. The scorched earth tactics used in Russia forced
Napoleon Napoleon Bonaparte ; it, Napoleone Bonaparte, ; co, Napulione Buonaparte. (born Napoleone Buonaparte; 15 August 1769 – 5 May 1821), later known by his regnal name Napoleon I, was a French military commander and political leader who ...
to withdraw his forces due to lack of food and shelter. Today, the
Law of land warfare The law of war is the component of international law that regulates the conditions for initiating war (''jus ad bellum'') and the conduct of warring parties (''jus in bello''). Laws of war define sovereignty and nationhood, states and territor ...
forbids looting and the confiscation of private property, but local supplies, particularly perishables, are still purchased when possible for use by occupying forces, and airplanes often use parachutes to drop supplies to besieged forces. Even as rules become stricter, the necessities of war become more numerous; in addition to food, shelter, and ammunition, today's militaries require fuel, batteries, spare mechanical parts, electronic equipment, and many other things. In the United States, the
Defense Logistics Agency The Defense Logistics Agency (DLA) is a combat support agency in the United States Department of Defense (DoD), with more than 26,000 civilian and military personnel throughout the world. Located in 48 states and 28 countries, DLA provides su ...
employs over 22,000 civilians with the sole task of logistics support, and 30,000 soldiers graduate from the U.S. Army Logistics Management College each year.


Communication

Another consideration is the importance of leadership being able to communicate with the invasion force. In ancient times, this often meant that a king needed to lead his armies in person to be certain his commands were timely and followed, as in the case of
Alexander the Great Alexander III of Macedon ( grc, Ἀλέξανδρος, Alexandros; 20/21 July 356 BC – 10/11 June 323 BC), commonly known as Alexander the Great, was a king of the ancient Greek kingdom of Macedon. He succeeded his father Philip II to ...
(356–323 BCE). At that time, the skills needed to lead troops in battle were as important as the skills needed to run a country during peacetime. When it was necessary for the king to be elsewhere, messengers would relay updates back to the rear, often on horseback or, in cases such as the
Battle of Marathon The Battle of Marathon took place in 490 BC during the first Persian invasion of Greece. It was fought between the citizens of Athens, aided by Plataea, and a Persian force commanded by Datis and Artaphernes. The battle was the culmination o ...
(490 BCE), with swift runners. When possible, sloops and cutters were used to relay information by sea. brought Britain the first news that
Nelson Nelson may refer to: Arts and entertainment * ''Nelson'' (1918 film), a historical film directed by Maurice Elvey * ''Nelson'' (1926 film), a historical film directed by Walter Summers * ''Nelson'' (opera), an opera by Lennox Berkeley to a lib ...
had defeated the French forces at the
Battle of Trafalgar The Battle of Trafalgar (21 October 1805) was a naval engagement between the British Royal Navy and the combined fleets of the French and Spanish Navies during the War of the Third Coalition (August–December 1805) of the Napoleonic Wars (180 ...
in 1805. The development of Morse Code, and later of voice communications by radio and satellite, have allowed even small units of skirmishers to remain in contact with a larger invasion force, to verify orders or to call for artillery support and air strikes. These communications were critical to the German blitzkrieg strategy, as infantry commanders relayed defensive positions to tanks and bombers.


Public relations

In diplomatic, public-relations and propaganda terms, it may help an invader (or a potential invader) to have an invitation as an excuse to intervene with a view to "restoring order" or "righting wrongs". Dissident groups, fifth columns or official circles may conspire to "call in" foreign assistance. Cases include: * From the point of view of the
Byzantine Empire The Byzantine Empire, also referred to as the Eastern Roman Empire or Byzantium, was the continuation of the Roman Empire primarily in its eastern provinces during Late Antiquity and the Middle Ages, when its capital city was Constantinopl ...
, the
First Crusade The First Crusade (1096–1099) was the first of a series of religious wars, or Crusades, initiated, supported and at times directed by the Latin Church in the medieval period. The objective was the recovery of the Holy Land from Islamic r ...
's invasion and conquest of the Levant (1096–1099) resulted from an invitation issued by Emperor
Alexios I Komnenos Alexios I Komnenos ( grc-gre, Ἀλέξιος Κομνηνός, 1057 – 15 August 1118; Latinized Alexius I Comnenus) was Byzantine emperor from 1081 to 1118. Although he was not the first emperor of the Komnenian dynasty, it was during ...
in 1095, seeking assistance against the Turks in Anatolia. * English barons opposed to King
John John is a common English name and surname: * John (given name) * John (surname) John may also refer to: New Testament Works * Gospel of John, a title often shortened to John * First Epistle of John, often shortened to 1 John * Secon ...
invited the French Prince
Louis Louis may refer to: * Louis (coin) * Louis (given name), origin and several individuals with this name * Louis (surname) * Louis (singer), Serbian singer * HMS ''Louis'', two ships of the Royal Navy See also Derived or associated terms * Lewis ( ...
to undertake a French invasion of England in the
First Barons' War The First Barons' War (1215–1217) was a civil war in the Kingdom of England in which a group of rebellious major landowners (commonly referred to as barons) led by Robert Fitzwalter waged war against King John of England. The conflict resulte ...
of 1215–1217. * In 1688 an
invitation to William The ''Invitation to William'' was a letter sent by seven notable English nobles, later called "the Immortal Seven", to stadtholder William III, Prince of Orange, received by him on 30 June 1688 (Julian calendar, 10 July Gregorian calendar). In ...
of Orange to invade Britain helped along the Glorious Revolution. *
Wolfe Tone Theobald Wolfe Tone, posthumously known as Wolfe Tone ( ga, Bhulbh Teón; 20 June 176319 November 1798), was a leading Irish revolutionary figure and one of the founding members in Belfast and Dublin of the United Irishmen, a republican soci ...
asked for French intervention (the unsuccessful
Expédition d'Irlande The French expedition to Ireland, known in French as the ''Expédition d'Irlande'' ("Expedition to Ireland"), was an unsuccessful attempt by the French Republic to assist the outlawed Society of United Irishmen, a popular rebel Irish republican ...
, 1796) in the lead-up to the
Irish Rebellion of 1798 The Irish Rebellion of 1798 ( ga, Éirí Amach 1798; Ulster-Scots: ''The Hurries'') was a major uprising against British rule in Ireland. The main organising force was the Society of United Irishmen, a republican revolutionary group influenced ...
. * Some members of the
Communist Party of Czechoslovakia The Communist Party of Czechoslovakia ( Czech and Slovak: ''Komunistická strana Československa'', KSČ) was a communist and Marxist–Leninist political party in Czechoslovakia that existed between 1921 and 1992. It was a member of the Comint ...
allegedly called for Soviet intervention during the Prague Spring of 1968, which ended with the
Warsaw Pact invasion of Czechoslovakia The Warsaw Pact invasion of Czechoslovakia refers to the events of 20–21 August 1968, when the Czechoslovak Socialist Republic was jointly invaded by four Warsaw Pact countries: the Soviet Union, the Polish People's Republic, the People's Rep ...
in August 1968. National foundation-legends can echo the theme of inviting foreign warriors to come and rule a people: note the traditional account in the '' Tale of Bygone Years'' of how
Varangian The Varangians (; non, Væringjar; gkm, Βάραγγοι, ''Várangoi'';Varangian
" Online Etymo ...
invaders came establish long-term rule in Novgorod (and subsequently throughout
Russia Russia (, , ), or the Russian Federation, is a transcontinental country spanning Eastern Europe and Northern Asia. It is the largest country in the world, with its internationally recognised territory covering , and encompassing one-eig ...
).


Applications regarding non-state combatants

In the 20th and 21st centuries, questions arose regarding the effectiveness of the invasion strategy in neutralizing non-state combatants, a type of warfare sometimes referred to as "
fourth generation warfare Fourth-generation warfare (4GW) is conflict characterized by a blurring of the lines between war and politics, combatants and civilians. The term was first used in 1980 by a team of United States analysts, including William S. Lind, to describe ...
". In this case, one or more combatant groups are controlled not by a centralized state government but by independent leadership, and these groups may be made up of civilians, foreign agents, mercenaries, politicians, religious leaders, and members of the regular military. These groups act in smaller numbers, are not confined by borders, and do not necessarily depend on the direct support of the state. Groups such as these are not easily defeated by straightforward invasion, or even constant occupation; the country's regular army may be defeated, the government may be replaced, but
asymmetric warfare Asymmetric warfare (or asymmetric engagement) is the term given to describe a type of war between belligerents whose relative military power, strategy or tactics differ significantly. This is typically a war between a standing, professional ar ...
on the part of these groups can be continued indefinitely. Because regular armed forces units do not have the flexibility and independence of small
covert cell A clandestine cell system is a method for organizing a group of people (such as resistance fighters, sleeper agents, mobsters, or terrorists) such that such people can more effectively resist penetration by an opposing organization (such as l ...
s, many believe that the concept of a powerful occupying force actually creates a disadvantage. An opposing theory holds that, in response to extremist ideology and unjust governments, an invasion can change the government and reeducate the people, making prolonged resistance unlikely and averting future violence. This theory acknowledges that these changes may take time—generations, in some cases—but holds that immediate benefits may still be won by reducing membership in, and choking the supply lines of, these covert cells. Proponents of the invasion strategy in such conflicts maintain the belief that a strong occupying force can still succeed in its goals on a tactical level, building upon numerous small victories, similar to a war of attrition. Contemporary debate on this issue is still fresh; neither side can claim to know for certain which strategies will ultimately be effective in defeating non-state combatants. Opponents of the invasion strategy point to a lack of examples in which occupying or peacekeeping forces have met with conclusive success. They also cite continuing conflicts such as
Northern Ireland Northern Ireland ( ga, Tuaisceart Éireann ; sco, label= Ulster-Scots, Norlin Airlann) is a part of the United Kingdom, situated in the north-east of the island of Ireland, that is variously described as a country, province or region. Nort ...
,
Israel Israel (; he, יִשְׂרָאֵל, ; ar, إِسْرَائِيل, ), officially the State of Israel ( he, מְדִינַת יִשְׂרָאֵל, label=none, translit=Medīnat Yīsrāʾēl; ), is a country in Western Asia. It is situated ...
, Chechnya, and
Iraq Iraq,; ku, عێراق, translit=Êraq officially the Republic of Iraq, '; ku, کۆماری عێراق, translit=Komarî Êraq is a country in Western Asia. It is bordered by Turkey to the north, Iran to the east, the Persian Gulf and K ...
, as well as examples which they claim ultimately proved to be failures, such as
Lebanon Lebanon ( , ar, لُبْنَان, translit=lubnān, ), officially the Republic of Lebanon () or the Lebanese Republic, is a country in Western Asia. It is located between Syria to Lebanon–Syria border, the north and east and Israel to Blue ...
, and
Afghanistan Afghanistan, officially the Islamic Emirate of Afghanistan,; prs, امارت اسلامی افغانستان is a landlocked country located at the crossroads of Central Asia and South Asia. Referred to as the Heart of Asia, it is bordere ...
. Supporters of the invasion strategy hold that it is too soon to call those situations failures, and that patience is needed to see the plan through. Some say that the invasions themselves have, in fact, been successful, but that political opponents and the international media skew the facts for sensationalism or political gain.


Outcomes

The outcomes of an invasion may vary according to the objectives of both invaders and defenders, the success of the invasion and the defense, and the presence or absence of an agreed settlement between the warring parties. The most common outcome is the loss of territory, generally accompanied by a change in government and often the loss of direct control of that government by the losing faction. This sometimes results in the transformation of that country into a
client state A client state, in international relations, is a state that is economically, politically, and/or militarily subordinate to another more powerful state (called the "controlling state"). A client state may variously be described as satellite state, ...
, often accompanied by requirements to pay
reparations Reparation(s) may refer to: Christianity * Restitution (theology), the Christian doctrine calling for reparation * Acts of reparation, prayers for repairing the damages of sin History *War reparations **World War I reparations, made from G ...
or
tribute A tribute (; from Latin ''tributum'', "contribution") is wealth, often in kind, that a party gives to another as a sign of submission, allegiance or respect. Various ancient states exacted tribute from the rulers of land which the state conqu ...
to the victor. In other cases the results of a successful invasion may simply be a return to the status quo; this can be seen in wars of attrition, when the destruction of personnel and supplies is the main strategic objective, or where a nation previously subdued and currently occupied by an aggressive third party is restored to control of its own affairs (i.e. Western Europe following the Normandy landings in 1944, or Kuwait following the defeat of Iraq in 1991). In some cases, the invasion may be strategically limited to a geographical area, which is carved into a separate state as with the Bangladesh Liberation War in 1971.


See also

* Aggression * Combat *
Conquest Conquest is the act of military subjugation of an enemy by force of arms. Military history provides many examples of conquest: the Roman conquest of Britain, the Mauryan conquest of Afghanistan and of vast areas of the Indian subcontinent, t ...
*
Invasion literature Invasion literature (also the invasion novel) is a literary genre that was popular in the period between 1871 and the First World War (1914–1918). The invasion novel first was recognized as a literary genre in the UK, with the novella '' The ...
*
List of invasions This is a list of invasions ordered by date. An invasion is a military offensive in which sizable number of combatants of one geopolitical entity aggressively enter territory controlled by another such entity, generally with the objectives of es ...
*
Military occupation Military occupation, also known as belligerent occupation or simply occupation, is the effective military control by a ruling power over a territory that is outside of that power's sovereign territory.Eyāl Benveniśtî. The international law ...
* Military rule * Power projection *
Sovereignty Sovereignty is the defining authority within individual consciousness, social construct, or territory. Sovereignty entails hierarchy within the state, as well as external autonomy for states. In any state, sovereignty is assigned to the perso ...
*
Trespass Trespass is an area of tort law broadly divided into three groups: trespass to the person, trespass to chattels, and trespass to land. Trespass to the person historically involved six separate trespasses: threats, assault, battery, woundi ...
*
War of aggression A war of aggression, sometimes also war of conquest, is a military conflict waged without the justification of self-defense, usually for territorial gain and subjugation. Wars without international legality (i.e. not out of self-defense nor san ...


References

{{Authority control Wars by type