World War I
Names: Before World War II, the events of 1914–1918 were generally known as the Great War or simply the World War. In August 1914, the magazine The Independent 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 unparalleled scale, devastation, and loss of life. The first recorded use of the term First World War was in September 1914 by German biologist and philosopher Ernst Haeckel who stated, "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."

Background: 

Political and military alliances: For much of the 19th century, the major European powers maintained a tenuous balance of power, known as the Concert of Europe. After 1848, this was challenged by Britain's withdrawal into so-called splendid isolation, the decline of the Ottoman Empire, New Imperialism, and the rise of Prussia under Otto von Bismarck. Victory in the 1870–1871 Franco-Prussian War allowed Bismarck to consolidate a German Empire. Post-1871, the primary aim of French policy was to avenge this defeat, but by the early 1890s, this had switched to the expansion of the French colonial empire.
In 1873, Bismarck negotiated the League of the Three Emperors, which included Austria-Hungary, Russia and Germany. After the 1877–1878 Russo-Turkish War, the League was dissolved due to Austrian concerns over the expansion of Russian influence in the Balkans, an area they considered to be 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 resolve any disputes between themselves. In 1887, Bismarck set up the Reinsurance Treaty, a secret agreement between Germany and Russia to remain neutral if either were attacked by France or Austria-Hungary.

For Bismarck, peace with Russia was the foundation of German foreign policy but in 1890, he was forced to retire by Wilhelm II. The latter was persuaded not to renew the Reinsurance Treaty by his new Chancellor, Leo von Caprivi. This gave France an opening to agree the Franco-Russian Alliance in 1894, which was then followed by the 1904 Entente Cordiale with Britain. The Triple Entente was completed by the 1907 Anglo-Russian Convention. While not formal alliances, by settling long-standing colonial disputes in Asia and Africa, British support for France or Russia in any future conflict became a possibility. This was accentuated by British and Russian support for France against Germany during the 1911 Agadir Crisis.

Arms race: German economic and industrial strength continued to expand rapidly post-1871. Backed by Wilhelm II, Admiral Alfred von Tirpitz sought to use this growth to build an Imperial German Navy, that could compete with the British Royal Navy. This policy was based on the work of US naval author Alfred Thayer Mahan, who argued that 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 surpass it. Bismarck thought that the British would not interfere in Europe, as 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 began. Despite the vast sums spent by Tirpitz, the launch of HMS Dreadnought in 1906 gave the British a technological advantage. 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 by German concern over Russia's quick recovery from its defeat in the Russo-Japanese War and subsequent 1905 Russian Revolution. Economic reforms led to a significant post-1908 expansion of railways and transportation 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 were taken by the Balkan powers and Italy, which led to increased expenditure by the Ottomans and Austria-Hungary. Absolute figures are difficult to calculate due to differences in categorising expenditure since they often omit civilian infrastructure projects like railways which had logistical importance and military use. It is known, however, that from 1908 to 1913, military 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 the 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 to be controlled by a weak Ottoman government, rather than an ambitious Slav power like Bulgaria. Russia had ambitions in northeastern Anatolia while its clients had overlapping claims in the Balkans. These competing interests divided Russian policy-makers and added to regional instability.
Austrian statesmen viewed the Balkans as essential for the continued existence of their Empire and saw 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 resolve the situation. Some historians see this as a significant escalation, ending any chance of Austria cooperating with Russia in the Balkans, while also damaging diplomatic relations between Serbia and Italy.
Tensions increased after the 1911–1912 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 overran most of the Ottomans' territory in the Balkans during the 1912–1913 First Balkan War, much to the surprise of outside observers. The Serbian capture of ports on the Adriatic resulted in partial Austrian mobilisation, starting on 21 November 1912, including units along the Russian border in Galicia. The Russian government decided not to mobilise in response, unprepared to precipitate a war.
The Great Powers sought to re-assert control through the 1913 Treaty of London, which had 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 I of Austria, visited Sarajevo, the capital of the recently annexed Bosnia and Herzegovina. Cvjetko Popović, Gavrilo Princip, Nedeljko Čabrinović, Trifko Grabež, Vaso Čubrilović (Bosnian Serbs) and Muhamed Mehmedbašić (from the Bosniaks community), from the movement known as Young Bosnia, took up positions along the Archduke's motorcade route, to assassinate him. Supplied with arms by extremists within the Serbian Black Hand intelligence organisation, they hoped his death would free Bosnia from Austrian rule.
Čabrinović threw a grenade at the Archduke's car and injured two of his aides. The other assassins were also unsuccessful. An hour later, as Ferdinand was returning from visiting the injured officers in hospital, his car took a wrong turn into a street where Gavrilo Princip was standing. He fired two pistol shots, fatally wounding Ferdinand and his wife Sophie.
According to historian Zbyněk Zeman, in Vienna "the event almost failed to make any impression whatsoever.