Intensified submarine warfare was the form of submarine warfare practiced by Germany in the first months of 1916. It was intended as a political compromise between the internationally-recognised Prize Rules, under which submarines were virtually ineffective as commerce raiders, and unrestricted submarine warfare in which merchant ships operating in designated War Zones were liable to be sunk without warning, and without provision for the safety of the passengers or crew. The policy was abandoned in May 1916 due to U.S. political pressure arising from a number of incidents, the most notable being the sinking of the cross-channel ferry Sussex.