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Ottoman Empire / Turkey
joined Oct 31, 1914
Triple Alliance/Central Powers
Germany
joined Aug 1, 1914
Triple Alliance/Central Powers
Austria-Hungary
joined July 28, 1914
Triple Alliance/Central Powers
United Kingdom
joined Aug 4, 1914
Triple Entente/Allies
Russia
joined July 26, 1914
Triple Entente/Allies
Serbia
joined July 28, 1914
Allied with Russia
Italy
joined May 23 1915 on the side of the Triple Entente/Allies
Originally partnered with central powers, but declared war on Austria-Hungary on the side of the Triple Entente/Allies
France
joined Aug 3, 1914
Triple Entente/Allies
Belgium
joined Aug 4, 1914
started neutral, joined the Triple Entente/Allies when invaded by Germany