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France
Germany declared war on France on August 3, 1914, causing them to be on the side of the allied powers. -
Russia
russia had to join after promising support to its Balkan ally Serbia against Austria-Hungary. By the end of 1917, the Bolsheviks had taken power in the major cities of Russia and introduced communist rule in those areas it controlled -
Great Britain
since Belgium's ports were nearby to the British coast and German control of Belgium would have been seen as a threat to Britain, they joined the war on Belgium.
