(March 24, 1916), torpedoing of a French cross-channel passenger steamer, the Sussex, by a German submarine, leaving 80 casualties, including two Americans wounded. The attack prompted a U.S. threat to sever diplomatic relations. The German government responded with the so-called Sussex pledge (May 4, 1916), agreeing to give adequate warning before sinking merchant and passenger ships and to provide for the safety of passengers and crew. The pledge was upheld until February 1917, when unrestricted submarine warfare was resumed.