The United States initially pursued a policy of neutrality in World War I, hoping to remain allies with all sides. However, Germany sinking American supply ships like the Lusitania and intercepted messages revealing German plans caused the U.S. to enter the war in 1917. American involvement helped tip the scales in favor of the Allies, with U.S. troops fighting in major battles and inflicting over 320,000 casualties on Germany. This combined with the economic might of the U.S. caused Germany to collapse and the Allies to achieve victory by 1918.