Intensified submarine warfare, a form of submarine warfare practiced by Germany in the first months of 1916,^[1] represented a German political compromise between the internationally recognised Prize Rules (which made submarines virtually ineffective as commerce raiders) and unrestricted submarine warfare (in which submarines sink merchant ships operating in designated War Zones without warning, and without provision for the safety of passengers or crew). Germany abandoned the policy in May 1916 due to U.S. political pressure arising from a number of incidents, most notably the torpedoing of the cross-channel ferry Sussex.^[2]