In May 1943, Allied forces used a newly adopted offensive doctrine and technological innovations to reverse the course of the Battle of the Atlantic. The British, followed by the Canadians, led the way in the North Atlantic. But the U.S. Navy also played a role there.
On 20 May, the 23 U-boats of Wolf Pack Mosel were assembling a patrol line in the North Atlantic air gap as westbound Convoy ON 184 approached. Protecting the 39-ship convoy was its close escort, Canadian Escort Group C-1, comprising the Canadian destroyers St. Lauren and St. Croix, the British frigate Itchen, and three Canadian corvettes. Augmenting these warships during the transit through the air gap was a support group whose assignment was offensive—to find and sink U-boats.