World War 1

World War I (28 July 1914 11 November 1918), often abbreviated as WWI, was one of the deadliest global conflicts in history. Belligerents included much of Europe, the Russian Empire, the United States, and the Ottoman Empire, with fighting occurring throughout Europe, the Middle East, Africa, the Pacific, and parts of Asia. An estimated 9 million soldiers were killed in combat, plus another 23 million wounded, while 5 million civilians died as a result of military action, hunger, and disease. Millions more died in genocides within the Ottoman Empire and in the 1918 influenza pandemic, which was exacerbated by the movement of combatants during the war.

Prior to 1914, the European great powers were divided between the Triple Entente (comprising France, Russia, and Britain) and the Triple Alliance (containing Germany, Austria-Hungary, and Italy). Tensions in the Balkans came to a head on 28 June 1914, following the assassination of Archduke Franz Ferdinand of Austria-Hungary by Gavrilo Princip, a Bosnian Serb. Austria-Hungary blamed Serbia, which led to the July Crisis, an unsuccessful attempt to avoid conflict through diplomacy. On 28 July 1914, Austria-Hungary declared war on Serbia, while Russia came to the latter's defence. By 4 August, Germany, France, and Britain (along with their respective colonies) were also drawn into the war. In November 1914, the Ottoman Empire, Germany, and Austria-Hungary formed the Central Powers, and on 26 April 1915, Italy joined Britain, France, Russia, and Serbia as the Allies of World War I.

The German strategy in 1914 was to concentrate its forces on defeating France in six weeks, before moving them to the Eastern Front and doing the same to Russia. However, this was defeated at the Marne in September 1914, and the year ended with the two sides facing each other along the Western Front, a continuous series of trenches stretching from the English Channel to Switzerland. The frontlines in the West changed little until 1917, while the Eastern Front was far more fluid, with both Austria-Hungary and Russia gaining and losing large swathes of territory. Other significant theatres included the Middle East, Italy, Asia Pacific, and the Balkans, which drew Bulgaria, Romania, and Greece into the war. Both Russia and Austria-Hungary suffered enormous casualties in the East throughout 1915, while Allied offensives in Gallipoli and the Western Front ended in failure. In 1916, German attacks at Verdun, and a Franco-British offensive on the Somme, led to heavy losses for limited strategic gains, while the Russian Brusilov offensive ground to a halt after early success. By 1917, Russia was on the verge of revolution, the French Nivelle offensive ended in failure, and British, French and German forces experienced heavy losses in Ypres, leaving all belligerents short of manpower and under severe economic stress. Shortages caused by the Allied naval blockade led Germany to initiate unrestricted submarine warfare, bringing the previously-neutral United States into the war on 6 April 1917.

In Russia, the Bolsheviks seized power in the 1917 October Revolution and exited the war with the March 1918 Treaty of Brest-Litovsk, freeing up large numbers of German troops. Germany used these additional resources to launch the March 1918 offensive, but was halted by stubborn Allied defence, heavy casualties, and supply shortages. When the Allies began the Hundred Days Offensive in August, the Imperial German Army continued to fight hard but could only slow the advance, not stop it. Towards the end of 1918, the Central Powers began to collapse; Bulgaria signed an armistice on 29 September, followed by the Ottomans on 31 October, then Austria-Hungary on 3 November. Isolated, facing the German Revolution at home and a military on the verge of mutiny, Kaiser Wilhelm II abdicated on 9 November, and the new German government signed the Armistice of 11 November 1918, bringing the conflict to a close. The Paris Peace Conference of 1919–1920 imposed various settlements on the defeated powers, with the best-known of these being the Treaty of Versailles. The dissolution of the Russian Empire in 1917, the German Empire in 1918, the Austria-Hungarian Empire in 1920, and the Ottoman Empire in 1922, led to numerous uprisings and the creation of independent states, including Poland, Czechoslovakia, and Yugoslavia. For reasons that are still debated, failure to manage the instability that resulted from this upheaval during the interwar period ended with the outbreak of World War II in September 1939.

Names

The term ''world war'' was first coined in September 1914 by German biologist and philosopher Ernst Haeckel. He claimed that "there is no doubt that the course and character of the feared 'European War' ... will become the first world war in the full sense of the word," in '' The Indianapolis Star'' on 20 September 1914.

The term First World War (often abbreviated as WWI or WW1), had been used by Lt-Col. Charles à Court Repington, as a title for his memoirs (published in 1920); he had noted his discussion on the matter with a Major Johnstone of Harvard University in his diary entry of 10 September 1918.
Prior to World War II, the events of 1914–1918 were generally known as the Great War or simply the World War. In August 1914, '' The Independent'' magazine wrote "This is the Great War. It names itself". In October 1914, the Canadian magazine '' Maclean's'' similarly wrote, "Some wars name themselves. This is the Great War." Contemporary Europeans also referred to it as "the war to end war" and it was also described as "the war to end all wars" due to their perception of its then-unparalleled scale, devastation, and loss of life. After World WarII began in 1939, the terms became more standard, with British Empire historians, including Canadians, favouring "The First World War" and Americans "World WarI".

Background

Political and military alliances

For much of the 19th century, the major European powers maintained a tenuous balance of power among themselves, known as the Concert of Europe. After 1848, this was challenged by a variety of factors, including Britain's withdrawal into so-called splendid isolation, the decline of the Ottoman Empire and the rise of Prussia under Otto von Bismarck. The 1866 Austro-Prussian War established Prussian hegemony in Germany, while victory in the 1870–1871 Franco-Prussian War allowed Bismarck to consolidate the German states into a German Empire under Prussian leadership. Avenging the defeat of 1871, or revanchism, and recovering the provinces of Alsace-Lorraine became the principal objects of French policy for the next forty years.

In order to isolate France and avoid a war on two fronts, Bismarck negotiated the League of the Three Emperors (German: ''Dreikaiserbund'') between Austria-Hungary, Russia and Germany. After Russian victory in the 1877–1878 Russo-Turkish War, the League was dissolved due to Austrian concerns over Russian influence in the Balkans, an area they considered of vital strategic interest. Germany and Austria-Hungary then formed the 1879 Dual Alliance, which became the Triple Alliance when Italy joined in 1882. For Bismarck, the purpose of these agreements was to isolate France by ensuring the three Empires resolved any disputes between themselves; when this was threatened in 1880 by British and French attempts to negotiate directly with Russia, he reformed the League in 1881, which was renewed in 1883 and 1885. After the agreement lapsed in 1887, he replaced it with the Reinsurance Treaty, a secret agreement between Germany and Russia to remain neutral if either were attacked by France or Austria-Hungary.

Bismarck viewed peace with Russia as the foundation of German foreign policy but after becoming Kaiser in 1890, Wilhelm II forced him to retire and was persuaded not to renew the Reinsurance Treaty by Leo von Caprivi, his new Chancellor. This provided France an opportunity to counteract the Triple Alliance, by signing the Franco-Russian Alliance in 1894, followed by the 1904 '' Entente Cordiale'' with Britain, and the Triple Entente was completed by the 1907 Anglo-Russian Convention. While these were not formal alliances, by settling long-standing colonial disputes in Africa and Asia, British entry into any future conflict involving France or Russia became a possibility. British and Russian support for France against Germany during the Agadir Crisis in 1911 reinforced their relationship and increased Anglo-German estrangement, deepening the divisions that would erupt in 1914.

Arms race

German industrial strength significantly increased after 1871, driven by the creation of a unified Reich, French indemnity payments and the annexation of Alsace-Lorraine. Backed by Wilhelm II, Admiral Alfred von Tirpitz sought to use this growth in economic power to build a ''Kaiserliche Marine'', or Imperial German Navy, which could compete with the British Royal Navy for world naval supremacy. His thinking was influenced by US naval strategist Alfred Thayer Mahan, who argued possession of a blue-water navy was vital for global power projection; Tirpitz had his books translated into German, while Wilhelm made them required reading for his advisors and senior military personnel.

However, it was also an emotional decision, driven by Wilhelm's simultaneous admiration for the Royal Navy and desire to outdo it. Bismarck calculated Britain would not interfere in Europe so long as its maritime supremacy remained secure, but his dismissal in 1890 led to a change in policy and an Anglo-German naval arms race. Despite the vast sums spent by Tirpitz, the launch of in 1906 gave the British a technological advantage over their German rival which they never lost. Ultimately, the race diverted huge resources into creating a German navy large enough to antagonise Britain, but not defeat it; in 1911, Chancellor Theobald von Bethmann Hollweg acknowledged defeat, leading to the ''Rüstungswende'' or 'armaments turning point', when he switched expenditure from the navy to the army.

This decision was not driven by a reduction in political tensions, but German concern over Russia's recovery from defeat in the 1905 Russo-Japanese War and subsequent revolution. Economic reforms backed by French funding led to a significant post-1908 expansion of railways and infrastructure, particularly in its western border regions. Since Germany and Austria-Hungary relied on faster mobilisation to compensate for their numerical inferiority compared to Russia, the threat posed by the closing of this gap was more important than competing with the Royal Navy. After Germany expanded its standing army by 170,000 troops in 1913, France extended compulsory military service from two to three years; similar measures taken by the Balkan powers and Italy, which led to increased expenditure by the Ottomans and Austria-Hungary. Absolute figures are hard to calculate due to differences in categorising expenditure, since they often omit civilian infrastructure projects like railways which also had a military use. However, from 1908 to 1913, defence spending by the six major European powers increased by over 50% in real terms.

Conflicts in the Balkans

The years before 1914 were marked by a series of crises in the Balkans as other powers sought to benefit from Ottoman decline. While Pan-Slavic and Orthodox Russia considered itself the protector of Serbia and other Slav states, they preferred the strategically vital Bosporus straits be controlled by a weak Ottoman government, rather than an ambitious Slav power like Bulgaria. Since Russia had its own ambitions in Eastern Turkey and their clients had over-lapping claims in the Balkans, balancing them divided Russian policy makers and added to regional instability.

Austrian statesmen viewed the Balkans as essential for the continued existence of their Empire and Serbian expansion as a direct threat. The 1908–1909 Bosnian Crisis began when Austria annexed the former Ottoman territory of Bosnia and Herzegovina, which it had occupied since 1878. Timed to coincide with the Bulgarian Declaration of Independence from the Ottoman Empire, this unilateral action was denounced by the European powers but accepted as there was no consensus on how to reverse it. Some historians see this as a significant escalation, ending any chance of Austria co-operating with Russia in the Balkans while damaging relations with Serbia and Italy, both of whom had their own expansionist ambitions in the area.

Tensions increased after the 1911 to 1912 Italo-Turkish War demonstrated Ottoman weakness and led to the formation of the Balkan League, an alliance of Serbia, Bulgaria, Montenegro, and Greece. The League quickly over-ran most of European Turkey in the 1912 to 1913 First Balkan War, much to the surprise of outside observers. The Serbian capture of ports on the Adriatic resulted in partial Austrian mobilisation on 21 November 1912, including units along the Russian border in Galicia. In a meeting the next day, the Russian government decided not to mobilise in response, unwilling to precipitate a war for which they were not yet prepared.

The Great Powers sought to re-assert control through the 1913 Treaty of London, which created an independent Albania, while enlarging the territories of Bulgaria, Serbia, Montenegro and Greece. However, disputes between the victors sparked the 33-day Second Balkan War, when Bulgaria attacked Serbia and Greece on 16 June 1913; it was defeated, losing most of Macedonia to Serbia and Greece, and Southern Dobruja to Romania. The result was that even countries which benefited from the Balkan Wars, such as Serbia and Greece, felt cheated of their "rightful gains", while for Austria it demonstrated the apparent indifference with which other powers viewed their concerns, including Germany. This complex mix of resentment, nationalism and insecurity helps explain why the pre-1914 Balkans became known as the "powder keg of Europe".

Prelude

Sarajevo assassination

On 28 June 1914, Archduke Franz Ferdinand of Austria, heir presumptive to Emperor Franz Joseph, visited Sarajevo, capital of the recently annexed provinces of Bosnia and Herzegovina. Six assassins from the movement known as Young Bosnia, or ''Mlada Bosna'', took up positions along the route taken by the Archduke's motorcade, with the intention of assassinating him. Supplied with arms by extremists within the Serbian Black Hand intelligence organisation, they hoped his death would free Bosnia from Austrian rule, although there was little agreement on what would replace it.

Nedeljko Čabrinović threw a grenade at the Archduke's car and injured two of his aides, who were taken to hospital while the convoy carried on. The other assassins were also unsuccessful but an hour later, as Ferdinand was returning from visiting the injured officers, his car took a wrong turn into a street where Gavrilo Princip was standing. He stepped forward and fired two pistol shots, fatally wounding Ferdinand and his wife Sophie, who both died shortly thereafter. Although Emperor Franz Joseph was shocked by the incident, political and personal differences meant the two men were not close; allegedly, his first reported comment was "A higher power has re-established the order which I, alas, could not preserve".

According to historian Zbyněk Zeman, his reaction was reflected more broadly in Vienna, where "the event almost failed to make any impression whatsoever. On Sunday 28 June and Monday 29th, the crowds listened to music and drank wine, as if nothing had happened." Nevertheless, the impact of the murder of the heir to the throne was significant, and has been described by historian Christopher Clark as a " 9/11 effect, a terrorist event charged with historic meaning, transforming the political chemistry in Vienna".

Expansion of violence in Bosnia and Herzegovina

The Austro-Hungarian authorities encouraged the subsequent anti-Serb riots in Sarajevo, in which Bosnian Croats and Bosniaks killed two Bosnian Serbs and damaged numerous Serb-owned buildings. Violent actions against ethnic Serbs were also organised outside Sarajevo, in other cities in Austro-Hungarian-controlled Bosnia and Herzegovina, Croatia and Slovenia. Austro-Hungarian authorities in Bosnia and Herzegovina imprisoned and extradited approximately 5,500 prominent Serbs, 700 to 2,200 of whom died in prison. A further 460 Serbs were sentenced to death. A predominantly Bosniak special militia known as the ''Schutzkorps'' was established and carried out the persecution of Serbs.

July Crisis

The assassination initiated the July Crisis, a month of diplomatic manoeuvring between Austria-Hungary, Germany, Russia, France and Britain. Believing Serbian intelligence helped organise Franz Ferdinand's murder, Austrian officials wanted to use the opportunity to end their interference in Bosnia and saw war as the best way of achieving this. However, the Foreign Ministry had no solid proof of Serbian involvement and a dossier used to make its case was riddled with errors. On 23July, Austria delivered an ultimatum to Serbia, listing ten demands made intentionally unacceptable to provide an excuse for starting hostilities.

Serbia ordered general mobilisation on 25July, but accepted all the terms, except for those empowering Austrian representatives to suppress "subversive elements" inside Serbia, and take part in the investigation and trial of Serbians linked to the assassination. Claiming this amounted to rejection, Austria broke off diplomatic relations and ordered partial mobilisation the next day; on 28 July, they declared war on Serbia and began shelling Belgrade. Having initiated war preparations on 25 July, Russia now ordered general mobilisation in support of Serbia on 30th.

Anxious to ensure backing from the SDP political opposition by presenting Russia as the aggressor, Bethmann-Hollweg delayed commencement of war preparations until 31 July. That afternoon the Russian government were handed a note requiring them to "cease all war measures against Germany and Austria-Hungary" within 12 hours. A further German demand for neutrality was refused by the French who ordered general mobilisation but delayed declaring war. The German General Staff had long assumed they faced a war on two fronts; the Schlieffen Plan envisaged using 80% of the army to defeat France in the west, then switch to Russia. Since this required them to move quickly, mobilisation orders were issued that afternoon.

At a meeting on 29 July, the British cabinet had narrowly decided its obligations to Belgium under the 1839 Treaty of London did not require it to oppose a German invasion with military force. However, this was largely driven by Prime Minister Asquith's desire to maintain unity; he and his senior Cabinet ministers were already committed to support France, the Royal Navy had been mobilised and public opinion was strongly in favour of intervention. On 31 July, Britain sent notes to Germany and France, asking them to respect Belgian neutrality; France pledged to do so, Germany did not reply.

Once the German ultimatum to Russia expired on the morning of 1 August, the two countries were at war. Later the same day, Wilhelm was informed by his Ambassador in London, Prince Lichnowsky, that Britain would remain neutral if France was not attacked, and might not intervene at all given the ongoing Home Rule Crisis in Ireland. Jubilant at this news, he ordered General Moltke, the German chief of staff, to "march the whole of the... army to the East". This allegedly brought Moltke to the verge of a nervous breakdown, who protested that "it cannot be done. The deployment of millions cannot be improvised." Lichnowsky soon realised he was mistaken, although Wilhelm insisted on waiting for a telegram from his cousin George V; once received it confirmed there had been a misunderstanding, and he told Moltke "Now do what you want."

Aware of German plans to attack through Belgium, French Commander-in-Chief Joseph Joffre asked his government for permission to cross the border and pre-empt such a move. To avoid a violation of Belgian neutrality, he was told any advance could come only after a German invasion. On 2 August, Germany occupied Luxembourg and exchanged fire with French units; on 3August, they declared war on France and demanded free passage across Belgium, which was refused. Early on the morning of 4August, the Germans invaded and Albert I of Belgium called for assistance under the Treaty of London. Britain sent Germany an ultimatum demanding they withdraw from Belgium; when this expired at midnight without a response, the two empires were at war.

Progress of the war

Opening hostilities

Confusion among the Central Powers

The strategy of the Central Powers suffered from miscommunication. Germany had promised to support Austria-Hungary's invasion of Serbia, but interpretations of what this meant differed. Previously tested deployment plans had been replaced early in 1914, but those had never been tested in exercises. Austro-Hungarian leaders believed Germany would cover its northern flank against Russia. Germany, however, envisioned Austria-Hungary directing most of its troops against Russia, while Germany dealt with France. This confusion forced the Austro-Hungarian Army to divide its forces between the Russian and Serbian fronts.

Serbian campaign

Beginning on 12 August, the Austrian and Serbs clashed at the battles of the Cer and Kolubara; over the next two weeks, Austrian attacks were repulsed with heavy losses, dashing their hopes of a swift victory and marking the first major Allied victories of the war. As a result, Austria had to keep sizeable forces on the Serbian front, weakening its efforts against Russia. Serbia's defeat of the 1914 invasion has been called one of the major upset victories of the twentieth century. In spring 1915, the campaign saw the first use of anti-aircraft warfare after an Austrian plane was shot down with ground-to-air fire, as well as the first medical evacuation by the Serbian army in autumn 1915.

German Offensive in Belgium and France

Upon mobilisation in 1914, 80% of the German Army was located on the Western Front, with the remainder acting as a screening force in the East; officially titled ''Aufmarsch II West,'' it is better known as the Schlieffen Plan after its creator, Alfred von Schlieffen, head of the German General Staff from 1891 to 1906. Rather than a direct attack across their shared frontier, the German right wing would sweep through the Netherlands and Belgium, then swing south, encircling Paris and trapping the French army against the Swiss border. Schlieffen estimated this would take six weeks, after which the German army would transfer to the East and defeat the Russians.

The plan was substantially modified by his successor, Helmuth von Moltke the Younger. Under Schlieffen, 85% of German forces in the west were assigned to the right wing, with the remainder holding along the frontier. By keeping his left wing deliberately weak, he hoped to lure the French into an offensive into the "lost provinces" of Alsace-Lorraine, which was in fact the strategy envisaged by their Plan XVII. However, Moltke grew concerned the French might push too hard on his left flank and as the German Army increased in size from 1908 to 1914, he changed the allocation of forces between the two wings from 85:15 to 70:30. He also considered Dutch neutrality essential for German trade and cancelled the incursion into the Netherlands, which meant any delays in Belgium threatened the entire viability of the plan. Historian Richard Holmes argues these changes meant the right wing was not strong enough to achieve decisive success and thus led to unrealistic goals and timings.

The initial German advance in the West was very successful and by the end of August the Allied left, which included the British Expeditionary Force (BEF), was in full retreat. At the same time, the French offensive in Alsace-Lorraine was a disastrous failure, with casualties exceeding 260,000, including 27,000 killed on 22 August during the Battle of the Frontiers. German planning provided broad strategic instructions, while allowing army commanders considerable freedom in carrying them out at the front; this worked well in 1866 and 1870 but in 1914, von Kluck used this freedom to disobey orders, opening a gap between the German armies as they closed on Paris. The French and British exploited this gap to halt the German advance east of Paris at the First Battle of the Marne from 5to 12 September and push the German forces back some .

In 1911, the Russian Stavka had agreed with the French to attack Germany within fifteen days of mobilisation, ten days before the Germans had anticipated, although it meant the two Russian armies that entered East Prussia on 17 August did so without many of their support elements. Although the Russian Second Army was effectively destroyed at the Battle of Tannenberg on 26–30 August, their advance caused the Germans to re-route their 8th Field Army from France to East Prussia, a factor in Allied victory on the Marne.

By the end of 1914, German troops held strong defensive positions inside France, controlled the bulk of France's domestic coalfields and had inflicted 230,000 more casualties than it lost itself. However, communications problems and questionable command decisions cost Germany the chance of a decisive outcome, while it had failed to achieve the primary objective of avoiding a long, two-front war. As was apparent to a number of German leaders, this amounted to a strategic defeat; shortly after the Marne, Crown Prince Wilhelm told an American reporter; "We have lost the war. It will go on for a long time but lost it is already."

Asia and the Pacific

On 30 August 1914, New Zealand occupied German Samoa, now the independent state of Samoa. On 11 September, the Australian Naval and Military Expeditionary Force landed on the island of New Britain, then part of German New Guinea. On 28 October, the German cruiser sank the Russian cruiser Zhemchug in the Battle of Penang. Japan declared war on Germany prior to seizing territories in the Pacific which later became the South Seas Mandate, as well as German Treaty ports on the Chinese Shandong peninsula at Tsingtao. After Vienna refused to withdraw its cruiser from Tsingtao, Japan declared war on Austria-Hungary as well, and the ship was sunk at Tsingtao in November 1914. Within a few months, Allied forces had seized all German territories in the Pacific, leaving only isolated commerce raiders and a few holdouts in New Guinea.

African campaigns

Some of the first clashes of the war involved British, French, and German colonial forces in Africa. On 6–7 August, French and British troops invaded the German protectorate of Togoland and Kamerun. On 10 August, German forces in South-West Africa attacked South Africa; sporadic and fierce fighting continued for the rest of the war. The German colonial forces in German East Africa, led by Colonel Paul von Lettow-Vorbeck, fought a guerrilla warfare campaign during World WarI and only surrendered two weeks after the armistice took effect in Europe.

Indian support for the Allies

Prior to the war, Germany had attempted to use Indian nationalism and pan-Islamism to its advantage, a policy continued post-1914 by instigating uprisings in India, while the Niedermayer–Hentig Expedition urged Afghanistan to join the war on the side of Central Powers. However, contrary to British fears of a revolt in India, the outbreak of the war saw a reduction in nationalist activity. This was largely because leaders from the Indian National Congress and other groups believed support for the British war effort would hasten Indian Home Rule, a promise allegedly made explicit in 1917 by Edwin Montagu, then Secretary of State for India.

In 1914, the British Indian Army was larger than the British Army itself, and between 1914 and 1918 an estimated 1.3 million Indian soldiers and labourers served in Europe, Africa, and the Middle East, while the Government of India and their princely allies supplied large quantities of food, money, and ammunition. In all, 140,000 soldiers served on the Western Front and nearly 700,000 in the Middle East, with 47,746 killed and 65,126 wounded.
The suffering engendered by the war, as well as the failure of the British government to grant self-government to India after the end of hostilities, bred disillusionment and fuelled the campaign for full independence that would be led by Mahatma Gandhi and others.

Western Front 1914 to 1916

Trench warfare begins

Pre-war military tactics that emphasised open warfare and the individual rifleman proved obsolete when confronted with conditions prevailing in 1914. Technological advances allowed the creation of strong defensive systems largely impervious to massed infantry advances, such as barbed wire, machine guns and above all far more powerful artillery, which dominated the battlefield and made crossing open ground extremely difficult. Both sides struggled to develop tactics for breaching entrenched positions without suffering heavy casualties. In time, however, technology began to produce new offensive weapons, such as gas warfare and the tank.

After the First Battle of the Marne in September 1914, Allied and German forces unsuccessfully tried to outflank each other, a series of manoeuvres later known as the " Race to the Sea". By the end of 1914, the opposing forces confronted each other along an uninterrupted line of entrenched positions from the Channel to the Swiss border. Since the Germans were normally able to choose where to stand, they generally held the high ground, while their trenches tended to be better built; those constructed by the French and English were initially considered "temporary", only needed until an offensive would smash the German defences. Both sides tried to break the stalemate using scientific and technological advances. On 22 April 1915, at the Second Battle of Ypres, the Germans (violating the Hague Convention) used chlorine gas for the first time on the Western Front. Several types of gas soon became widely used by both sides, and though it never proved a decisive, battle-winning weapon, it became one of the most-feared and best-remembered horrors of the war.

Continuation of trench warfare

Neither side proved able to deliver a decisive blow for the next two years. Throughout 1915–17, the British Empire and France suffered more casualties than Germany, because of both the strategic and tactical stances chosen by the sides. Strategically, while the Germans mounted only one major offensive, the Allies made several attempts to break through the German lines.

In February 1916 the Germans attacked French defensive positions at the Battle of Verdun, lasting until December 1916. The Germans made initial gains, before French counter-attacks returned matters to near their starting point. Casualties were greater for the French, but the Germans bled heavily as well, with anywhere from 700,000 to 975,000 casualties suffered between the two combatants. Verdun became a symbol of French determination and self-sacrifice.

The Battle of the Somme was an Anglo-French offensive of July to November 1916. The opening day on 1 July 1916 was the bloodiest single day in the history of the British Army, which suffered 57,470 casualties, including 19,240 dead. As a whole, the Somme offensive led to an estimated 420,000 British casualties, along with 200,000 French and 500,000 German. Gun fire was not the only factor taking lives; the diseases that emerged in the trenches were a major killer on both sides. The living conditions made it so that countless diseases and infections occurred, such as trench foot, shell shock, blindness/burns from mustard gas, lice, trench fever, " cooties" ( body lice) and the ' Spanish flu'.

Naval war

At the start of the war, German cruisers were scattered across the globe, some of which were subsequently used to attack Allied merchant shipping. The British Royal Navy systematically hunted them down, though not without some embarrassment from its inability to protect Allied shipping. For example, the light cruiser , which was part of the German East Asia Squadron stationed at Qingdao, seized or sank 15 merchantmen, as well as a Russian cruiser and a French destroyer. Most of the squadron was returning to Germany when it sank two British warships armoured cruisers at the Battle of Coronel in November 1914, before being virtually destroyed at the Battle of the Falkland Islands in December. The SMS Dresden escaped with a few auxiliaries, but after the Battle of Más a Tierra, these too had either been destroyed or interned.

Soon after the outbreak of hostilities, Britain began a naval blockade of Germany. The strategy proved effective, cutting off vital military and civilian supplies, although this blockade violated accepted international law codified by several international agreements of the past two centuries. Britain mined international waters to prevent any ships from entering entire sections of ocean, causing danger to even neutral ships. Since there was limited response to this tactic of the British, Germany expected a similar response to its unrestricted submarine warfare.

The Battle of Jutland (German: ''Skagerrakschlacht'', or "Battle of the Skagerrak") in May/June 1916 developed into the largest naval battle of the war. It was the only full-scale clash of battleships during the war, and one of the largest in history. The Kaiserliche Marine's High Seas Fleet, commanded by Vice Admiral Reinhard Scheer, fought the Royal Navy's Grand Fleet, led by Admiral Sir John Jellicoe. The engagement was a stand off, as the Germans were outmanoeuvred by the larger British fleet, but managed to escape and inflicted more damage to the British fleet than they received. Strategically, however, the British asserted their control of the sea, and the bulk of the German surface fleet remained confined to port for the duration of the war.

German U-boats attempted to cut the supply lines between North America and Britain. The nature of submarine warfare meant that attacks often came without warning, giving the crews of the merchant ships little hope of survival. The United States launched a protest, and Germany changed its rules of engagement. After the sinking of the passenger ship RMS ''Lusitania'' in 1915, Germany promised not to target passenger liners, while Britain armed its merchant ships, placing them beyond the protection of the " cruiser rules", which demanded warning and movement of crews to "a place of safety" (a standard that lifeboats did not meet). Finally, in early 1917, Germany adopted a policy of unrestricted submarine warfare, realising the Americans would eventually enter the war. Germany sought to strangle Allied sea lanes before the United States could transport a large army overseas, but after initial successes eventually failed to do so.

The U-boat threat lessened in 1917, when merchant ships began travelling in convoys, escorted by destroyers. This tactic made it difficult for U-boats to find targets, which significantly lessened losses; after the hydrophone and depth charges were introduced, accompanying destroyers could attack a submerged submarine with some hope of success. Convoys slowed the flow of supplies since ships had to wait as convoys were assembled. The solution to the delays was an extensive program of building new freighters. Troopships were too fast for the submarines and did not travel the North Atlantic in convoys. The U-boats had sunk more than 5,000 Allied ships, at a cost of 199 submarines.

World War I also saw the first use of aircraft carriers in combat, with launching Sopwith Camels in a successful raid against the Zeppelin hangars at Tondern in July 1918, as well as blimps for antisubmarine patrol.

Southern theatres

War in the Balkans

Faced with Russia in the east, Austria-Hungary could spare only one-third of its army to attack Serbia. After suffering heavy losses, the Austrians briefly occupied the Serbian capital, Belgrade. A Serbian counter-attack in the Battle of Kolubara succeeded in driving them from the country by the end of 1914. For the first ten months of 1915, Austria-Hungary used most of its military reserves to fight Italy. German and Austro-Hungarian diplomats, however, scored a coup by persuading Bulgaria to join the attack on Serbia. The Austro-Hungarian provinces of Slovenia, Croatia and Bosnia provided troops for Austria-Hungary in the fight with Serbia, Russia and Italy. Montenegro allied itself with Serbia.

Bulgaria declared war on Serbia on 14 October 1915 and joined in the attack by the Austro-Hungarian army under Mackensen's army of 250,000 that was already underway. Serbia was conquered in a little more than a month, as the Central Powers, now including Bulgaria, sent in 600,000 troops total. The Serbian army, fighting on two fronts and facing certain defeat, retreated into northern Albania. The Serbs suffered defeat in the Battle of Kosovo. Montenegro covered the Serbian retreat towards the Adriatic coast in the Battle of Mojkovac in 6–7 January 1916, but ultimately the Austrians also conquered Montenegro. The surviving Serbian soldiers were evacuated by ship to Greece. After conquest, Serbia was divided between Austro-Hungary and Bulgaria.

In late 1915, a Franco-British force landed at Salonica in Greece to offer assistance and to pressure its government to declare war against the Central Powers. However, the pro-German King Constantine I dismissed the pro-Allied government of Eleftherios Venizelos before the Allied expeditionary force arrived. The friction between the King of Greece and the Allies continued to accumulate with the National Schism, which effectively divided Greece between regions still loyal to the king and the new provisional government of Venizelos in Salonica. After intense negotiations and an armed confrontation in Athens between Allied and royalist forces (an incident known as Noemvriana), the King of Greece resigned and his second son Alexander took his place; Greece officially joined the war on the side of the Allies in June 1917.

The Macedonian front was initially mostly static. French and Serbian forces retook limited areas of Macedonia by recapturing Bitola on 19 November 1916 following the costly Monastir offensive, which brought stabilisation of the front.

Serbian and French troops finally made a breakthrough in September 1918 in the Vardar offensive, after most of the German and Austro-Hungarian troops had been withdrawn. The Bulgarians were defeated at the Battle of Dobro Pole, and by 25 September British and French troops had crossed the border into Bulgaria proper as the Bulgarian army collapsed. Bulgaria capitulated four days later, on 29 September 1918. The German high command responded by despatching troops to hold the line, but these forces were far too weak to re-establish a front.

The disappearance of the Macedonian front meant that the road to Budapest and Vienna was now opened to Allied forces. Hindenburg and Ludendorff concluded that the strategic and operational balance had now shifted decidedly against the Central Powers and, a day after the Bulgarian collapse, insisted on an immediate peace settlement.

Ottoman Empire

The Ottomans threatened Russia's Caucasian territories and Britain's communications with India via the Suez Canal. As the conflict progressed, the Ottoman Empire took advantage of the European powers' preoccupation with the war and conducted large-scale ethnic cleansing of the indigenous Armenian, Greek, and Assyrian Christian populations, known as the Armenian genocide, Greek genocide, and Assyrian genocide.

The British and French opened overseas fronts with the Gallipoli (1915) and Mesopotamian campaigns (1914). In Gallipoli, the Ottoman Empire successfully repelled the British, French, and Australian and New Zealand Army Corps (ANZACs). In Mesopotamia, by contrast, after the defeat of the British defenders in the siege of Kut by the Ottomans (1915–16), British Imperial forces reorganised and captured Baghdad in March 1917. The British were aided in Mesopotamia by local Arab and Assyrian fighters, while the Ottomans employed local Kurdish and Turcoman tribes.

Further to the west, the Suez Canal was defended from Ottoman attacks in 1915 and 1916; in August, a German and Ottoman force was defeated at the Battle of Romani by the ANZAC Mounted Division and the 52nd (Lowland) Infantry Division. Following this victory, an Egyptian Expeditionary Force advanced across the Sinai Peninsula, pushing Ottoman forces back in the Battle of Magdhaba in December and the Battle of Rafa on the border between the Egyptian Sinai and Ottoman Palestine in January 1917.

Russian armies generally had success in the Caucasus campaign. Enver Pasha, supreme commander of the Ottoman armed forces, was ambitious and dreamed of re-conquering central Asia and areas that had been lost to Russia previously. He was, however, a poor commander. He launched an offensive against the Russians in the Caucasus in December 1914 with 100,000 troops, insisting on a frontal attack against mountainous Russian positions in winter. He lost 86% of his force at the Battle of Sarikamish.

The Ottoman Empire, with German support, invaded Persia (modern Iran) in December 1914 in an effort to cut off British and Russian access to petroleum reservoirs around Baku near the Caspian Sea. Persia, ostensibly neutral, had long been under the spheres of British and Russian influence. The Ottomans and Germans were aided by Kurdish and Azeri forces, together with a large number of major Iranian tribes, such as the Qashqai, Tangistanis, Lurs, and Khamseh, while the Russians and British had the support of Armenian and Assyrian forces. The Persian campaign was to last until 1918 and end in failure for the Ottomans and their allies. However, the Russian withdrawal from the war in 1917 led to Armenian and Assyrian forces, who had hitherto inflicted a series of defeats upon the forces of the Ottomans and their allies, being cut off from supply lines, outnumbered, outgunned and isolated, forcing them to fight and flee towards British lines in northern Mesopotamia.

General Yudenich, the Russian commander from 1915 to 1916, drove the Turks out of most of the southern