At the beginning of the war, Admiral Karl Dönitz, commander of the German submarine fleet, had only 46 U-boats (submarines) under his command. By September 1942, the German fleet had expanded to an operational strength of 382 U-boats. The Germans employed Rudeltaktiks, a strategy in which U-boats took up positions along the main convoy routes. Once a U-boat had sighted a convoy, the captain would radio its coordinates to the German Submarine Headquarters, which would then direct other U-boats in the area to converge on the Allied ships. The U-boats conducted their attacks from the surface under cover of darkness. Although the Royal Navy (RN) and Royal Canadian Navy (RCN) escorts were equipped with ASDIC (a type of sonar that used soundings to determine the range and depth of submarines), it was only effective for locating submerged U-boats.
The British and Canadians referred to the German U-boat units as "wolf packs," and the German tactics proved highly effective. However, once RN and RCN destroyers and corvettes were equipped with radar in January 1942, they were able to detect U-boats on the surface. As a result, they were able to take the offensive against the German wolf packs. As the Battle of the Atlantic progressed, German U- boat losses would begin to mount. By the fall of 1943, Allied convoys were able to cross the Atlantic with far fewer losses.
By January 1942, the RCN had increased it corvette fleet to 64, and 60 more would be added in the next two years. At the same time, the RCN would also acquire additional destroyers and frigates. By March 1944, the RCN would assume complete responsibility for all North Atlantic convoys. On 22 July 1944, the RCN assembled a force of one frigate, six corvettes, and three light aircraft carriers to escort convoy HXS-300, a fleet composed of 167 Canadian, British, and American merchant vessels. The convoy did not lose a single ship during the Atlantic crossing.