The U-Boat Menace:
Hundreds of defenseless merchant ships were brutally attacked by U-boats. In response, the Allies formed convoys, consisting of up to 200 merchant ships, which were escorted across the Atlantic by escort carriers and destroyers. When a U-boat attacked a convoy, the destroyers launched explosive underwater depth charges and/or hedgehog bombs in an effort to thwart the submerged submarine.
Hitler countered by forming groups of U-boats, called Wolfpacks, to wreak havoc on the convoys. In March 1943, the largest Wolfpack of the war — more than 40 U-boats strong — attacked two convoys made up of 100 Allied ships. Twenty-one merchant ships were sent to the bottom.