Up until America entered the war, they had been supporting both sides through trade and financial support, but they helped the Allies the more than the Central Powers. Germany retaliated against America and the Allies by using unrestricted submarine warfare. They would attack and sink any ship within designated areas, and it didn't matter what kind of ship it was, who was on it, or what was on it. These U-boats were extremely important to their navy, and they used it as a way to starve out Britain. One boat that was destroyed through this warfare was the Lusitania, and the U.S. was not happy about it. This caused Germany to end the naval warfare on May 7, 1915, for some time because of the fear of America joining the war. The number of U-boats grew from 20 to 140 in a few years, and they returned to their policy in 1917. Germany's choice of war was also a big factor in U.S. involvement.