The Germans torpedoed the U.S. Gulflight without warning despite America's neutrality. While the German government apologised, they would not alter their policy of unrestricted submarine warfare.
Editors. "U.S. vessel Gulflight sunk by German submarines". The Literary Encyclopedia. First published 01 March 2010
[http://www.litencyc.com/php/stopics.php?rec=true&UID=12290, accessed 12 February 2016.]