During the Battle of the Atlantic, British merchant shipping was formed into convoys for protection against German submarine attack. In March 1943 convoys HX 229 and SC 122 were the focus of the largest convoy battle of the war. Kriegsmarine tactics against convoys employed multiple-submarine wolfpack tactics in nearly simultaneous surface attacks at night. Patrolling aircraft restricted the ability of submarines to converge on convoys during daylight. The North Atlantic winters offered the longest periods of darkness to conceal surfaced submarine operations. The winter of 1942–43 saw the largest number of submarines deployed to the mid-Atlantic before comprehensive anti-submarine aircraft patrols could be extended into that area.

Mid-Ocean Escort Force (MOEF) referred to the organisation of anti-submarine escorts for World War II trade convoys between Canada and Newfoundland, and the British Isles. The allocation of United States, British and Canadian escorts to these convoys reflected preferences of the United States upon United States' declaration of war and the organisation persisted through the winter of 1942–43 despite withdrawal of United States ships from the escort groups. By the summer of 1943, United States Atlantic escorts were focused on the faster CU convoys and the UG convoys between Chesapeake Bay and the Mediterranean Sea; and only British and Canadian escorts remained on the HX, SC and ON convoys.