World War I

World War I (WWI) was a global war centred in Europe that began on 28 July 1914 and lasted until 11 November 1918. It was predominantly called the World War or the Great War from its occurrence until the start of World War II in 1939, and the First World War or World War I thereafter. 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 technological advancements that led to enormous increases in the lethality of weapons without corresponding improvements in protection or mobility. It was the sixth-deadliest conflict in world history, subsequently paving the way for various political changes such as revolutions in many of 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 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 from East Prussia and Poland 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, the Allies drove back the German armies in a series of successful offensives and United States forces began entering the trenches. 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 agreed to be factors contributing to World War II.