World War I, known then as the Great War and referred to as "The War To End All Wars," was a global military conflict which took place primarily in Europe between 1914 and 1918. More than nine million soldiers and civilians died. The conflict had a decisive impact on the history of the 20th century.
The Allied Powers, led by France, Russia, the United Kingdom, 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 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. These fortifications ran from the North Sea to Switzerland, a distance of 475 miles, 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 East 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 overseas empire and states such as Czechoslovakia, Estonia, Finland, Latvia, Lithuania, Poland and Yugoslavia gained independence. 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.