 Complete the following phrase:  German submarines torpedoed ships without warning, causing sailors and passengers to drown. Berlin explained that submarines were so vulnerable that they dared not surface near merchant ships that might be carrying guns and which were too small to rescue submarine crews. Britain armed most of its merchant ships with medium caliber guns that could sink a submarine, making above-water attacks too risky. In February 1915, the United States warned Germany about the misuse of submarines. On April 22, the German Imperial Embassy warned U.S. citizens against boarding vessels to Britain, which would have to face German attack. On May 7, Germany torpedoed the British passenger liner 