Between September 1915–January 1917, the Germans practised restricted use of their U-boats. This meant that instead of issuing warnings of imminent attack and targeting all ships, they only attacked warships. The restriction was imposed after several cruise liners were sunk by U-boats including the US’s cruise ship, Lusitania, on 7 May 1915. The Germans were warned by the US that if they failed to impose restrictions, diplomatic ties would be severed. The Germans agreed and imposed such severe constraints on U-boat operations, that the use of these vessels dropped considerably. German submarine commanders, angered by the restrictions, insisted that the U-boats should be unrestricted. During the five months preceding derestriction, the Germans practised ‘intensified’ U-boat activity and unofficially restarted unrestricted warfare. The German military position had weakened and it was felt that unrestricted use of U-boats could aid German victory.
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