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 "RMS Lusitania", sinking her. This act of aggression caused the loss of 1,198 civilian lives, including 128 Americans. The sinking of a large, unarmed passenger ship, combined with the previous stories of atrocities in Belgium, shocked Americans and turned public opinion hostile to Germany, although not yet to the point of war. Wilson issued a warning to Germany that it would face "strict accountability" if it sank more neutral U.S. passenger ships.&lt;ref name="http://www.firstworldwar.com/wilsonwarning"&gt;&lt;/ref&gt; Berlin acquiesced, ordering its submarines to avoid passenger ships.