An invasion is a
military offensive in which large numbers of
combatants of one
geopolitical entity aggressively enter
territory 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 or gaining concessions from said government; or a combination thereof. An invasion can be the cause of a
war, 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
Strategy (from Greek στρατηγία ''stratēgia'', "art of troop leader; office of general, command, generalship") is a general plan to achieve one or more long-term or overall goals under conditions of uncertainty. In the sense of the "art ...
in planning and execution.
History
Archaeological
Archaeology or archeology is the scientific study of human activity through the recovery and analysis of material culture. The archaeological record consists of artifacts, architecture, biofacts or ecofacts, sites, and cultural landscap ...
evidence indicates that invasions have been frequent occurrences since
prehistory. In antiquity, before
radio communications and fast
transportation, 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,
philosophy
Philosophy (from , ) is the systematized study of general and fundamental questions, such as those about existence, reason, knowledge, values, mind, and language. Such questions are often posed as problems to be studied or resolved. Some ...
, and
technology that shaped the development of much of the
ancient world.
Before the days of package tours and cut-price airlines, military invasions functioned as a standard major form of proto-
tourism – 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
rivers,
marshes, or rugged
terrain, these measures have historically included
fortifications. 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,
Hadrian's Wall
Hadrian's Wall ( la, Vallum Aelium), also known as the Roman Wall, Picts' Wall, or ''Vallum Hadriani'' in Latin, is a former defensive fortification of the Roman province of Britannia, begun in AD 122 in the reign of the Emperor Hadrian. R ...
, and the
Danewerk are famous examples. Such barriers have also included
trench lines and, in more modern times,
minefield
A land mine is an explosive device concealed under or on the ground and designed to destroy or disable enemy targets, ranging from combatants to vehicles and tanks, as they pass over or near it. Such a device is typically detonated automati ...
s,
cameras, and
motion-sensitive sensor
A sensor is a device that produces an output signal for the purpose of sensing a physical phenomenon.
In the broadest definition, a sensor is a device, module, machine, or subsystem that detects events or changes in its environment and sends ...
s. 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 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,
Allied forces used airdropped mines to severely disrupt
Japanese logistical operations within their own borders.
Alternatively, the fortifications can be built up at a series of sites, such as
castle
A castle is a type of fortified structure built during the Middle Ages predominantly by the nobility or royalty and by military orders. Scholars debate the scope of the word ''castle'', but usually consider it to be the private fortified r ...
s 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
The Maginot Line (french: Ligne Maginot, ), named after the French Minister of War André Maginot, is a line of concrete fortifications, obstacles and weapon installations built by France in the 1930s to deter invasion by Germany and force the ...
. Forts can be positioned so that the
garrison
A garrison (from the French ''garnison'', itself from the verb ''garnir'', "to equip") is any body of troops stationed in a particular location, originally to guard it. The term now often applies to certain facilities that constitute a mil ...
s can interdict the
supply lines of the invaders. The theory behind these spaced forts is that the invader cannot afford to bypass these defenses, and so must lay
siege 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 have made lighter, more mobile defenses desirable to military planners. The obsolescence of large fortifications was displayed by the failure of the
Maginot Line
The Maginot Line (french: Ligne Maginot, ), named after the French Minister of War André Maginot, is a line of concrete fortifications, obstacles and weapon installations built by France in the 1930s to deter invasion by Germany and force the ...
in the beginning of World War Two. Nations defending against modern invasions normally use large population centers such as
cities or
towns as defensive points. The invader must capture these points to destroy the defender's ability to wage war. The defender uses mobile
armored and
infantry 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
The Iraqi Ground Forces (Arabic: القوات البرية العراقية), or the Iraqi Army (Arabic: الجيش العراقي), is the ground force component of the Iraqi Armed Forces. It was known as the Royal Iraqi Army up until the coup ...
'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,
Tikrit, and
Basra in the major combat in the
Iraq War. A defender can also use these mobile assets to precipitate a counteroffensive like the
Soviet Red Army at the
Battle of Kursk or the
Northern Alliance in
Afghanistan.
However, static emplacements remain useful in both defense against naval attacks and
defense against air attacks.
Naval mines are still an inexpensive but effective way to defend
ports and choke off supply lines. Large static air defense systems that combine antiaircraft guns with
missile launchers are still the best way to defend against air attacks. Such systems were used effectively by the
North Vietnamese around
Hanoi. Also, the
United States has invested considerable time and money into the construction of a
National Missile Defense system, a static defense grid intended to intercept nuclear
intercontinental ballistic missile
An intercontinental ballistic missile (ICBM) is a ballistic missile with a range greater than , primarily designed for nuclear weapons delivery (delivering one or more thermonuclear warheads). Conventional, chemical, and biological weapons c ...
s.
Island nation
An island country, island state or an island nation is a country whose primary territory consists of one or more islands or parts of islands. Approximately 25% of all independent countries are island countries. Island countries are historically ...
s, such as the
United Kingdom or
Japan
Japan ( ja, 日本, or , and formally , ''Nihonkoku'') is an island country in East Asia. It is situated in the northwest Pacific Ocean, and is bordered on the west by the Sea of Japan, while extending from the Sea of Okhotsk in the north ...
, and continental states with extensive
coasts, such as the United States, have utilized a significant
naval
A navy, naval force, or maritime force is the branch of a nation's armed forces principally designated for naval warfare, naval and amphibious warfare; namely, lake-borne, riverine, littoral zone, littoral, or ocean-borne combat operations and ...
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
Volgograd ( rus, Волгогра́д, a=ru-Volgograd.ogg, p=vəɫɡɐˈɡrat), geographical renaming, formerly Tsaritsyn (russian: Цари́цын, Tsarítsyn, label=none; ) (1589–1925), and Stalingrad (russian: Сталингра́д, Stal ...
. It can also cause the invading force to extend too far, allowing a
pincer movement to cut them off from reinforcements. This was the cause of the British defeat at the
Battle of Cowpens during the
American Revolutionary War. 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
The Second Punic War (218 to 201 BC) was the second of three wars fought between Carthage and Rome, the two main powers of the western Mediterranean in the 3rd century BC. For 17 years the two states struggled for supremacy, primarily in Ital ...
.
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