American Military History, Volume 2 by Richard W. Stewart

Even as the Allied ground campaign was proceeding on the shores of the Mediterranean, three other campaigns were under way from the British Isles: the campaign of the U.S. and the Royal Navies to defeat the German submarine; a U.S.-British strategic bombing offensive against Germany; and, intricately tied in with the other two, a logistical marathon to assemble the men and tools necessary for a direct assault.

Most critical of all was the antisubmarine campaign, for without success in that the two others could progress only feebly at best. The turning point in that campaign came in the spring of 1943, when the full effect of all the various devices used against the U-boat began to appear. Despite the subsequent German in- troduction of an acoustical torpedo that homed on the noise of an escort's propellers and later of the schnorkel, a steel tube extending above water by means of which the U-boat could charge its batteries without surfacing, Allied shipping losses continued to decline. In the last two years of the war the submarines would sink only one- seventh of the shipping they had in the earlier years.