The cruiser submarine concept originated during the unrestricted submarine warfare campaign of 1917. Three German Type U 139 submarines and seven former merchant submarines, each armed with two 15-centimetre (5.9 in) guns, patrolled areas distant from their North Sea bases to sink Allied merchant shipping as part of an effort to end World War I by starving the United Kingdom of Great Britain and Ireland. These distant patrols enjoyed unique immunity to the defensive convoy measures which limited successful submarine attacks in the vicinity of the British Isles.[2]