World War I (abbreviated WWI), also known as the First World War, the Great War and The War to End all Wars was a global military conflict that took place mostly in Europe between 1914 and 1918. The main combatants were the Allied Powers, led by France, the Russian Empire, the British Empire, Serbia, Belgium, and later Italy, Romania and the United States, who fought against the Central Powers: Austria-Hungary, the German Empire, Bulgaria, and the Ottoman Empire (present day Turkey).
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. More than nine million soldiers died on the various battlefields, and millions more civilians perished.
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 and 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.