World War I (often abbreviated as WWI or WW1), also known as the First World War or the Great War, was a global conflict originating in Europe that lasted from July 28, 1914 to November 11, 1918. Contemporaneously described as "the war to end all wars," it led to the mobilization of more than 70 million military personnel, including 60 million Europeans, making it one of the largest wars in history. It was also one of the deadliest conflicts in history, with an estimated nine million combatants and seven million civilian deaths as a direct result of the war, while resulting genocides and the 1918 influenza pandemic lead to another 50 to 100 million deaths worldwide. 

The war caused the disintegration of four empires: the Austro-Hungarian, German, Ottoman, and Russian. Germany lost its overseas empire, and states such as Czechoslovakiaand Yugoslavia were created, or recreated, as in the cases of Lithuania and Poland. This contributed to a decisive break with the world order that had emerged after the Napoleonic Wars, which was modified by the mid-19th century's nationalistic revolutions. The results of World War I would also be important factors in the development of World War II just over two decades later.

Much of the fighting in World War I took place along the Western Front, within a system of opposing manned trenches and fortifications (separated by a "no man's land") running from the North Sea to the border of Switzerland. On the Eastern Front, the vast eastern plains and limited rail network prevented a trench warfare stalemate from developing, although the scale of the conflict was just as large. Hostilities also occurred on and under the sea and - for the first time - in the air.

WORLD WAR ONE KILLING FIELDS, by Cotter Bass, provides a brief but graphic overview of the battlefield horrors of World War I.