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Usa declared war against germany because the germans sunk an American ship Lusitania.
America entered World War One on April 6th, 1917. Up to that date, America had tried to keep out of World War One – though she had traded with nations involved in the war – but unrestricted submarine warfare, introduced by the Germans on January 9th, 1917, was the primary issue that caused Woodrow Wilson to ask Congress to declare war on Germany on April 2nd. Four days later, America joined World War One on the side of the Allies.
Subject: Social studies
Topic: World war1 and 2