Germany decided to resume its unrestricted submarine warfare on February 1, 1917. The sinking of the Lusitania by a German U-boat on May 7, 1915 caused President Woodrow Wilson to demand that Germany stop attacking unarmed merchant ships. By September 1915 the operation of the U-boat warfare was totally suspended. The German naval commanders could not accept the decision and convinced the kaiser Wilhelm to support them in relaunching the U-boats for warfare.
