During the First World War, the UK blockaded Germany and armed its merchant ships to help defend them against U-boats. A single stern gun, equivalent to what a submarine might carry, was mounted; and civilian captains were encouraged to flee and shoot back from their more stable gun platform. 766 civilian ships had been armed by December 1915. Arming of merchant ships pushed Germany away from prize rules towards unrestricted submarine warfare, an important factor in bringing the United States into the war against Germany.