World War I, also known as the First World War or the Great War, was a global conflict fought from 1914 to 1918. It began in Europe but soon spread to parts of the Middle East, Africa, Asia, and the Pacific. The war was fought mainly between the Allies of World War I and the Central Powers, and involved the mobilization of tens of millions of soldiers from more than 40 states.
The immediate trigger was the Assassination of Archduke Franz Ferdinand in Sarajevo on 28 June 1914 by Gavrilo Princip. The crisis that followed led Austria-Hungary to declare war on Serbia, setting off a chain reaction among the major powers. Germany backed Austria-Hungary and then declared war on Russia and France; after the German invasion of Belgium, the United Kingdom entered the war. The conflict quickly widened into a world war through alliance systems and imperial rivalries.
On the Western Front (World War I), the war became defined by Trench warfare, artillery barrages, and stalemate. The Eastern Front (World War I) remained more mobile, while major campaigns also took place in the Balkans, the Middle East, and at sea. The war saw major military innovations, including widespread use of Aircraft, tanks, Machine guns, Chemical weapons, and Submarine warfare. It also caused the Armenian genocide and contributed to the spread of the Spanish flu pandemic.
The United States entered the war in 1917, while Russia withdrew after the October Revolution and the Treaty of Brest-Litovsk. By late 1918 the Central Powers were collapsing, and the war ended with the Armistice of 11 November 1918. The peace settlements, especially the Treaty of Versailles, redrew borders, weakened defeated empires, and helped create new states such as Poland, Czechoslovakia, and Yugoslavia. The war also led to the founding of the League of Nations and is widely seen as a major prelude to World War II.