The Allies needed to keep the vital flow of men and supplies going between North America and Europe, where they could be used in the fighting, while the Germans wanted to cut these supply lines. In order for them to do this, German submarines (u- boats) and other warships prowled the Atlantic Ocean sinking Allied transport ships.
Early in the war, German U-boats took a heavy toll on merchant shipping as the Allies struggled to find effective ways to combat the enemy threat. Between 1939 and 1942, the Germans increased the number of U-boats from 30 to 300 and developed effective hunting techniques like using groups of submarines, called wolfpacks, to attack convoys. Their efforts initially paid off, with 454,000 tonnes of shipping being lost to German U-boats in June 1941 alone. Their successes continued as nearly 400 Allied ships were sunk between January and July 1942, while only seven U-boats were lost. The situation was very serious for the Allies, as merchant ships were being sunk faster than they could be replaced, thereby putting the supply link between North America and Europe at great risk.
Aircraft were effective in protecting merchant ships, but the Allied planes used earlier in the war did not have enough range to offer air cover for the convoys all the way across the Atlantic.