World War I (or the First World War, often abbreviated as WWI or WW1) was a global war originating in Europe that lasted from 28 July 1914 to 11 November 1918. Contemporaneously known as the Great War or "the war to end all wars",[7] it led to the mobilisation of more than 70 million military personnel, including 60 million Europeans, making it one of the largest wars in history.[8][9] It is also one of the deadliest conflicts in history,[10] with an estimated nine million combatant deaths and 13 million civilian deaths as a direct result of the war,[11] while resulting genocides and the related 1918 Spanish flu pandemic caused another 17–100 million deaths worldwide,[12][13] including an estimated 2.64 million Spanish flu deaths in Europe and as many as 675,000 Spanish flu deaths in the United States.[14]

On 28 June 1914, Gavrilo Princip, a Bosnian Serb Yugoslav nationalist, assassinated the Austro-Hungarian heir Archduke Franz Ferdinand in Sarajevo, leading to the July Crisis.[15][16] In response, Austria-Hungary issued an ultimatum to Serbia on 23 July. Serbia's reply failed to satisfy the Austrians, and the two moved to a war footing. A network of interlocking alliances enlarged the crisis from a bilateral issue in the Balkans to one involving most of Europe. By July 1914, the great powers of Europe were divided into two coalitions: the Triple Entente, consisting of France, Russia, and Britain; and the Triple Alliance of Germany, Austria-Hungary, and Italy. The Triple Alliance was only defensive in nature, allowing Italy to stay out of the war until April 1915, when it joined the Allied Powers after its relations with Austria-Hungary deteriorated.[17] Russia felt it necessary to back Serbia, and approved partial mobilisation after Austria-Hungary shelled the Serbian capital of Belgrade on 28 July.[18] Full Russian mobilisation was announced on the evening of 30 July; the following day, Austria-Hungary and Germany did the same, while Germany demanded Russia demobilise within twelve hours.[19] When Russia failed to comply, Germany declared war on Russia on 1 August in support of Austria-Hungary, the latter following suit on 6 August; France ordered full mobilisation in support of Russia on 2 August.[20]

Germany's strategy for a war on two fronts against France and Russia was to rapidly concentrate the bulk of its army in the West to defeat France within six weeks, then shift forces to the East before Russia could fully mobilise; this was later known as the Schlieffen Plan.[21] On 2 August, Germany demanded free passage through Belgium, an essential element in achieving a quick victory over France.[22] When this was refused, German forces invaded Belgium on 3 August and declared war on France the same day; the Belgian government invoked the 1839 Treaty of London and, in compliance with its obligations under this treaty, Britain declared war on Germany on 4 August. On 12 August, Britain and France also declared war on Austria-Hungary; on 23 August, Japan sided with Britain, seizing German possessions in China and the Pacific. In November 1914, the Ottoman Empire entered the war on the side of Austria-Hungary and Germany, opening fronts in the Caucasus, Mesopotamia, and the Sinai Peninsula. The war was fought in (and drew upon) each power's colonial empire also, spreading the conflict to Africa and across the globe. The Entente and its allies eventually became known as the Allied Powers, while the grouping of Austria-Hungary, Germany and their allies became known as the Central Powers.

The German advance into France was halted at the Battle of the Marne and by the end of 1914, the Western Front settled into a war of attrition, marked by a long series of trench lines that changed little until 1917 (the Eastern Front, by contrast, was marked by much greater exchanges of territory). In 1915, Italy joined the Allied Powers and opened a front in the Alps. Bulgaria joined the Central Powers in 1915 and Greece joined the Allies in 1917, expanding the war in the Balkans. The United States initially remained neutral, though even while neutral it became an important supplier of war materiel to the Allies. Eventually, after the sinking of American merchant ships by German submarines, the declaration by Germany that its navy would resume unrestricted attacks on neutral shipping, and the revelation that Germany was trying to incite Mexico to initiate war against the United States, the U.S. declared war on Germany on 6 April 1917. Trained American forces did not begin arriving at the front in large numbers until mid-1918, but the American Expeditionary Force ultimately reached some two million troops.[23]

Though Serbia was defeated in 1915, and Romania joined the Allied Powers in 1916 only to be defeated in 1917, none of the great powers were knocked out of the war until 1918. The 1917 February Revolution in Russia replaced the Tsarist autocracy with the Provisional Government, but continuing discontent with the cost of the war led to the October Revolution, the creation of the Soviet Socialist Republic, and the signing of the Treaty of Brest-Litovsk by the new government in March 1918, ending Russia's involvement in the war. Germany now controlled much of eastern Europe and transferred large numbers of combat troops to the Western Front. Using new tactics, the German March 1918 Offensive was initially successful. The Allies fell back and held. The last of the German reserves were exhausted as 10,000 fresh American troops arrived every day. The Allies drove the Germans back in their Hundred Days Offensive, a continual series of attacks to which the Germans had no reply.[24] One by one the Central Powers quit. First Bulgaria, then the Ottoman Empire and the Austro-Hungarian empire. With its allies defeated, revolution at home, and the military no longer willing to fight, Kaiser Wilhelm abdicated on 9 November and Germany signed an armistice on 11 November 1918, ending the fighting.

World War I was a significant turning point in the political, cultural, economic, and social climate of the world. The war and its immediate aftermath sparked numerous revolutions and uprisings. The Big Four (Britain, France, the United States, and Italy) imposed their terms on the defeated powers in a series of treaties agreed at the 1919 Paris Peace Conference, the most well known being the German peace treaty: the Treaty of Versailles.[25] Ultimately, as a result of the war, the Austro-Hungarian, German, Ottoman, and Russian Empires ceased to exist, and numerous new states were created from their remains. However, despite the conclusive Allied victory (and the creation of the League of Nations during the Peace Conference, intended to prevent future wars), a second world war followed just over twenty years later.

Political and military alliances

For much of the 19th century, the major European powers had tried to maintain a tenuous balance of power among themselves, resulting in a complex network of political and military alliances.[32] The biggest challenges to this were Britain's withdrawal into so-called splendid isolation, the decline of the Ottoman Empire and the post-1848 rise of Prussia under Otto von Bismarck. Victory in the 1866 Austro-Prussian War established Prussian hegemony in Germany, while victory over France in the 1870–1871 Franco-Prussian War unified the German states into a German Reich under Prussian leadership. French desire for revenge over the defeat of 1871, known as revanchism, and the recovery of Alsace-Lorraine became a principal object of French policy for the next forty years (see French–German enmity).[33]

In 1873, 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. Concerned by Russia's victory in the 1877–1878 Russo-Turkish War and its influence in the Balkans, the League was dissolved in 1878, with Germany and Austria-Hungary subsequently forming the 1879 Dual Alliance; this became the Triple Alliance when Italy joined in 1882.[34][35]

The practical details of these alliances were limited, since their primary purpose was to ensure cooperation between the three Imperial Powers, and to isolate France. Attempts by Britain in 1880 to resolve colonial tensions with Russia and diplomatic moves by France led to Bismarck reforming the League in 1881.[36] When the League finally lapsed in 1887, it was replaced by the Reinsurance Treaty, a secret agreement between Germany and Russia to remain neutral if either were attacked by France or Austria-Hungary.

In 1890, the new German Emperor, Kaiser Wilhelm II, forced Bismarck to retire and was persuaded not to renew the Reinsurance Treaty by the new Chancellor, Leo von Caprivi.[37] This allowed France to counteract the Triple Alliance with the Franco-Russian Alliance of 1894 and the 1904 Entente Cordiale with Britain, while in 1907 Britain and Russia signed the Anglo-Russian Convention. The agreements did not constitute formal alliances, but by settling long-standing colonial disputes, they made British entry into any future conflict involving France or Russia a possibility. These interlocking bilateral agreements became known as the Triple Entente.[38] British backing of France against Germany during the Second Moroccan Crisis in 1911 reinforced the Entente between the two countries (and with Russia as well) and increased Anglo-German estrangement, deepening the divisions that would erupt in 1914.[39]

Arms race

The creation of the German Reich following victory in the 1871 Franco-Prussian War led to a massive increase in Germany's economic and industrial strength. Admiral Alfred von Tirpitz and Wilhelm II, who became Emperor in 1890, sought to use that to create a Kaiserliche Marine or Imperial German Navy to compete with Britain's Royal Navy for world naval supremacy.[40] In doing so, they were influenced by US naval strategist Alfred Mahan, who argued possession of a blue-water navy was vital for global power projection; Tirpitz translated his books into German and Wilhelm made them required reading.[41] However, it was also driven by Wilhelm's admiration of the Royal Navy and desire to outdo it.[42]

This resulted in the Anglo-German naval arms race but the launch of HMS Dreadnought in 1906 gave the Royal Navy a technological advantage over its German rival, which they never lost.[40] Ultimately, the race diverted huge resources to 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 Germany switched expenditure from the navy to the army.[43]

This was driven by Russia's recovery from the 1905 Revolution, specifically increased investment post 1908 in railways and infrastructure in its western border regions. Germany and Austria-Hungary relied on faster mobilisation to compensate for fewer numbers; it was concern at the closing of this gap that led to the end of the naval race, rather than a reduction in tension elsewhere. When Germany expanded its standing army by 170,000 men in 1913, France extended compulsory military service from two to three years; similar measures taken by the Balkan powers and Italy led to increased expenditure by the Ottomans and Austria-Hungary. Absolute figures are hard to calculate, due to differences in categorising expenditure, while they often omit civilian infrastructure projects with a military use, such as railways. However, from 1908 to 1913, defence spending by the six major European powers increased by over 50% in real terms.[44]

Conflicts in the Balkans

Sarajevo citizens reading a poster with the proclamation of the Austrian annexation in 1908

In October 1908, Austria-Hungary precipitated the Bosnian crisis of 1908–1909 by officially annexing the former Ottoman territory of Bosnia and Herzegovina, which it had occupied since 1878. This angered the Kingdom of Serbia and its patron, the Pan-Slavic and Orthodox Russian Empire. The Balkans came to be known as the "powder keg of Europe".[45] The Italo-Turkish War in the 1911–1912 was a significant precursor of the World War I as it sparked nationalism in the Balkan states and paved the way for the Balkan Wars.[46]

In 1912 and 1913, the First Balkan War was fought between the Balkan League and the fracturing Ottoman Empire. The resulting Treaty of London further shrank the Ottoman Empire, creating an independent Albanian state while enlarging the territorial holdings of Bulgaria, Serbia, Montenegro, and Greece. When Bulgaria attacked Serbia and Greece on 16 June 1913, it sparked the 33-day Second Balkan War, by the end of which it lost most of Macedonia to Serbia and Greece, and Southern Dobruja to Romania, further destabilising the region.[47] The Great Powers were able to keep these Balkan conflicts contained, but the next one would spread throughout Europe and beyond.