World War I, also known as the First World War, the Great War and the War To End All Wars, was a global military conflict which took place primarily in Europe from 1914 to 1918.^[2] Over 40 million casualties resulted, including approximately 20 million military and civilian deaths.^[3]

The Entente Powers, led by France, Russia, the British Empire, and later Italy (from 1915), and the United States (from 1917), defeated the Central Powers, led by the Austro-Hungarian, German, and Ottoman Empires. Russia withdrew from the war after its revolution in 1917.

The fighting that took place along the Western Front occurred along a system of trenches, breastworks, and fortifications separated by an area known as no man's land.^[4] These fortifications stretched 475 miles (more than 600 kilometres)^[4] and defined the war for many. On the Eastern Front, the vast eastern plains and limited rail network prevented a trench warfare stalemate, though the scale of the conflict was just as large as on the Western Front. The Middle Eastern Front and the Italian Front also saw heavy fighting, while hostilities also occurred at sea, and for the first time, in the air.

The war caused the disintegration of four empires: the Austro-Hungarian, German, Ottoman, and Russian. Germany lost its colonial empire; Czechoslovakia, Estonia, Finland, Latvia, Lithuania, and Poland gained independence, while the Kingdom of Yugoslavia came into existence as a successor to the Kingdom of Serbia. The cost of waging the war set the stage for the breakup of the British Empire as well and left France devastated for more than a generation.

World War I marked the end of the world order which had existed after the Napoleonic Wars, and was an important factor in the outbreak of World War II.