World War I (WWI), which was predominantly called the World War or the Great War from its occurrence until 1939, and the First World War or World War I thereafter, was a major war centred in Europe that began on 28 July 1914 and lasted until 11 November 1918. It involved all the world's great powers, which were assembled in two opposing alliances: the Allies (based on the Triple Entente of the United Kingdom,France and Russia) and the Central Powers (originally centred around the Triple Alliance of Germany, Austria-Hungary and Italy; but, as Austria–Hungary had taken the offensive against the agreement, Italy did not enter into the war). These alliances both reorganised (Italy fought for the Allies), and expanded as more nations entered the war. Ultimately more than 70 million military personnel, including 60 million Europeans, were mobilised in one of the largest wars in history. More than 9 million combatants were killed, largely because of great technological advances in firepower without corresponding advances in mobility. It was the sixth-deadliest conflict in world history, subsequently paving the way for various political changes such as revolutions in the nations involved.
Long-term causes of the war included the imperialistic foreign policies of the great powers of Europe, including the German Empire, the Austro-Hungarian Empire, the Ottoman Empire, the Russian Empire, the British Empire, the French Republic, and Italy. The assassination on 28 June 1914 of Archduke Franz Ferdinand of Austria, the heir to the throne of Austria-Hungary, by a Yugoslav nationalist in Sarajevo, Bosnia and Herzegovina was the proximate trigger of the war. It resulted in a Habsburg ultimatum against the Kingdom of Serbia.Several alliances formed over the previous decades were invoked, so within weeks the major powers were at war; via their colonies, the conflict soon spread around the world.
On 28 July, the conflict opened with the Austro-Hungarian invasion of Serbia, followed by the German invasion of Belgium, Luxembourg and France; and a Russian attack against Germany. After the German march on Paris was brought to a halt, the Western Front settled into a static battle of attrition with a trench line that changed little until 1917. In the East, the Russian army successfully fought against the Austro-Hungarian forces but was forced back by the German army. Additional fronts opened after the Ottoman Empire joined the war in 1914, Italy and Bulgaria in 1915 and Romania in 1916. The Russian Empire collapsed in March 1917, and Russia left the war after the October Revolution later that year. After a 1918 German offensive along the western front, United States forces entered the trenches and the Allies drove back the German armies in a series of successful offensives. Germany, which had its own trouble with revolutionaries at this point, agreed to a cease-fire on 11 November 1918, later known as Armistice Day. The war had ended in victory for the Allies.
Events on the home fronts were as tumultuous as on the battle fronts, as the participants tried to mobilize their manpower and economic resources to fight a total war. By the end of the war, four major imperial powers —the German, Russian, Austro-Hungarian and Ottoman empires— ceased to exist. The successor states of the former two lost a great amount of territory, while the latter two were dismantled entirely. The map of central Europe was redrawn into several smaller states. The League of Nations was formed in the hope of preventing another such conflict. The European nationalism spawned by the war and the breakup of empires, the repercussions of Germany's defeat and problems with the Treaty of Versailles are generally agreed to be factors contributing to World War II.
In the 19th century, the major European powers had gone to great lengths to maintain a balance of power throughout Europe, resulting by 1900 in a complex network of political and military alliances throughout the continent. These had started in 1815, with the Holy Alliance between Prussia, Russia, and Austria. Then, in October 1873, German Chancellor Bismarck negotiated the League of the Three Emperors (German: Dreikaiserbund) between the monarchs of Austria–Hungary, Russia and Germany. This agreement failed because Austria–Hungary and Russia could not agree over Balkan policy, leaving Germany and Austria–Hungary in an alliance formed in 1879, called the Dual Alliance. This was seen as a method of countering Russian influence in the Balkans as the Ottoman Empire continued to weaken. In 1882, this alliance was expanded to include Italy in what became the Triple Alliance.
After 1870, European conflict was averted largely through a carefully planned network of treaties between the German Empire and the remainder of Europe orchestrated by Bismarck. He especially worked to hold Russia at Germany's side to avoid a two-front war with France and Russia. When Wilhelm II ascended to the throne as German Emperor (Kaiser), Bismarck's alliances were gradually de-emphasised. For example, the Kaiser refused to renew the Reinsurance Treaty with Russia in 1890. Two years later, the Franco-Russian Alliance was signed to counteract the force of the Triple Alliance. In 1904, the United Kingdom sealed an alliance with France, the Entente Cordiale, and in 1907, the United Kingdom and Russia signed the Anglo-Russian Convention. This system of interlocking bilateral agreements formed theTriple Entente.
German industrial and economic power had grown greatly after unification and the foundation of the Empire in 1871. From the mid-1890s on, the government of Wilhelm II used this base to devote significant economic resources to building up the Kaiserliche Marine (Imperial German Navy), established by AdmiralAlfred von Tirpitz, in rivalry with the British Royal Navy for world naval supremacy.As a result, each nation strove to out-build the other in terms of capital ships. With the launch of HMS Dreadnought in 1906, the British Empire expanded on its significant advantage over its German rival.The arms race between Britain and Germany eventually extended to the rest of Europe, with all the major powers devoting their industrial base to producing the equipment and weapons necessary for a pan-European conflict.Between 1908 and 1913, the military spending of the European powers increased by 50 percent.
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, thePan-Slavic and Orthodox Russian Empire.Russian political manoeuvring in the region destabilised peace accords that were already fracturing in what was known as "the powder keg of Europe".
On 28 June 1914, Gavrilo Princip, a Bosnian-Serb student and member of Young Bosnia, assassinated the heir to the Austro-Hungarian throne, Archduke Franz Ferdinand of Austria in Sarajevo, Bosnia.This began a period of diplomatic manoeuvring among Austria-Hungary, Germany, Russia, France, and Britain called the July Crisis. Wanting to finally end Serbian interference in Bosnia, Austria-Hungary delivered the July Ultimatum to Serbia, a series of ten demands intentionally made unacceptable, intending to provoke a war with Serbia. When Serbia agreed to only eight of the ten demands, Austria-Hungary declared war on 28 July 1914. Strachan argues, "Whether an equivocal and early response by Serbia would have made any difference to Austria-Hungary's behaviour must be doubtful. Franz Ferdinand was not the sort of personality who commanded popularity, and his demise did not cast the empire into deepest mourning".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 both Serbia and Greece on 16 June 1913, it lost most of Macedonia to Serbia and Greece and Southern Dobrujato Romania in the 33-day Second Balkan War, further destabilising the region.
The Russian Empire, unwilling to allow Austria–Hungary to eliminate its influence in the Balkans, and in support of its longtime Serb protégés, ordered a partial mobilisation one day later. When the German Empire began to mobilise on 30 July 1914, France, resentful of the German conquest of Alsace-Lorraine during the Franco-Prussian War, ordered French mobilisation on 1 August. Germany declared war on Russia on the same day. The United Kingdom declared war on Germany on 4 August 1914, following an "unsatisfactory reply" to the British ultimatum that Belgium must be kept neutral.