Before 1914 the Great Powers were in two big alliance blocs: the Triple Alliance (Germany, Austria-Hungary and Italy) and the Triple Entente (France, Russia and Britain).
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The war extended and changed these two sides. Germany and its allies were known as the Central Powers: Germany and Austria-Hungary, later joined by the Ottoman Empire (Turkey plus the Middle East) and Bulgaria. The war quickly involved countries not part of the Triple Entente, so the opposing side was known as the Allies: Serbia, Russia, France and its Empire, Belgium, Montenegro and Britain and its Empire, including self-governing colonies like Canada and Australia.
Italy changed sides and joined the Allies in 1915. Other Allied nations included Portugal, Japan, Greece, Romania, China and, towards the end of the war, various South American countries, including Brazil and Peru.
The United States fought alongside the Allies from 1917, but as an ‘Associated Power’ with no formal military alliance.
Seán Lang is a senior lecturer in history at Anglia Ruskin University, and the author of First World War for Dummies.
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This answer was first published by History Extra in January 2016