World War I (abbreviated WWI), also known as the First World War, the Great War and The War to End all Wars was a
military conflict that took place mostly in
Europe between 1914 and 1918. The main combatants were the
Allied Powers, led by
Russian Empire, the
Belgium, and later
Romania and the
United States, who fought against the
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
Yugoslavia were created, or recreated, as in the cases of
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.