The United States did not enter World War I on a policy of neutrality. The US was neutral at the start of the war, but as the war progressed, the United States became increasingly involved in the conflict. The US entered World War I in 1917 after Germany announced a policy of unrestricted submarine warfare, which violated international law and posed a threat to American ships and citizens. In addition, the sinking of the British ocean liner Lusitania in 1915, in which 128 Americans died, further outraged American public opinion. These factors, combined with concerns about the balance of power in Europe and the potential for German victory, led the United States to abandon its policy of neutrality and enter the war on the side of the Allies in April 1917.