There were three main reaons that the US declared war on Germany in 1917,  There were three main reasons that the US declared war on Germany in 1917, which were all related to German actions in the war. 1) Germany's continuation of unrestricted submarine warfare to cut off supplies from Britain and France. This had resulted in the sinking of the liner Lusitania two years earlier, in 1915, that had killed American passengers. 2) The German foreign ministry had sent the Zimmerman Telegram instructing their embassy to seek alliance with Mexico. This message, intercepted by the British, provoked concern and even outrage in the US. 3) The US had strong economic and cultural ties to France and Great Britain. With the Germans negotiating a peace treaty with Russia (the USSR), it was feared that the British and French might be overwhelmed allowing Germany to control Europe.

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