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France
The French and the Russians had been in a military alliance for some time. ... Germany declared war on Russia on August 1st, 1914 when Russia refused to demobilize its army. The Germans knew that Russia would take longer to defeat, so France was the main target. -
Britain
Belgium's ports were close to the British coast and German control of Belgium would have been seen as a serious threat to Britain. In the end, Britain refused to ignore the events of 4 August 1914, when Germany attacked France through Belgium. Within hours, Britain declared war on Germany. -
Italy
Leading up to WWI, Italy had formed an alliance with the Central Powers of the German Empire and the Empire of the Austria-Hungary in the Triple Alliance. In 1915, Italy signed the secret Treaty of London and came into the war on the side of the Triple Entente. -
USA
In 1917, Germany, determined to win its war of attrition against the Allies, announced the resumption of unrestricted warfare in war-zone waters. Three days later, the United States broke diplomatic relations with Germany, and just hours after that the American liner Housatonic was sunk by a German U-boat.
