After careful consideration, the German government decided to wage unrestricted submarine warfare against Britain in February 1915. It took this decision primarily because of Britain's refusal to lift its naval blockade of Germany. The blockade contravened the 1909 Geneva Conventions on the conduct of naval warfare. German submarines attacked any shipping bound for British ports, including neutral vessels.
During the months of February and March 1917, German submarines sank several American merchant ships. As a result, the United States declared war on Germany on 6 April 1917. With the entry of the United States, the Allies acquired access to the vast resources of American industry and much- needed troops for the Western Front.