The French steamer Sussex provided ferry service on the English Channel. On a crossing to Dieppe in March 1916, the ship was mistaken for a minelayer and torpedoed by a German U-boat. The Sussex did not sink; it was towed into the French port of Boulogne. Fifty persons were killed in the incident; no Americans lost their lives, but several were injured.
President Wilson addressed the Congress in April and issued an ultimatum to the Germans: End the attack on unarmed ships or risk the severing of diplomatic relations.
Germany responded to Wilson's demands on May 4 with what is called the "Sussex Pledge." German submarine policy would henceforth be governed by promises to:
- end the sinking of passenger ships
- search merchant ships for contraband and make provisions for passengers and crews before sinking merchant ships
The German guarantees were generally honored until the announcement of the resumption of unrestricted submarine warfare in February 1917.
See discussion of U.S. policy and German submarine warfare.