World War I was a global war centered in Europe that began on 28 July 1914 and lasted until 11 November 1918. From the time of its occurrence until the approach of World War II, it was called simply the World War or the Great War, and thereafter the First World War or World War I. In America, it is also often referred to as the European War.

More than 70 million military personnel, including 60 million Europeans, were mobilized 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 second deadliest conflict in Western history, after the Thirty Years’ War (1618–1648).

The war is commonly said to have begun on 28 June 1914, with the assassination of Archduke Franz Ferdinand of Austria, by Bosnian Serb Gavrilo Princip in Sarajevo. This set off a diplomatic crisis when Austria-Hungary delivered an ultimatum to the Kingdom of Serbia. Within weeks, the major powers were at war and World War I had begun.

As more and more young men flocked to enlist, armies grew larger and larger. Trench warfare became commonplace, with both sides dug in for months on end, fighting a bloody stalemate. New technologies such as tanks, chemical weapons, and aircraft changed the nature of warfare, while the global conflict drew in more and more countries, eventually involving every major power.

The war finally ended on 11 November 1918, when an armistice between the Allies and Germany went into effect.