The Battle of the Atlantic widened after the United States 'joined the war in December 1941. Until then, U-boats had attacked only British ships. Now they also attacked American shipping on the US east coast, in the Caribbean and then in the Gulf of Mexico, sinking more than 200 ships before the US Navy began its own convoy system. Checked by the convoys, the U-boats moved for a while into the South Atlantic, attacking ships off the African coast taking goods to Britain from the Far East.

As the Battle of the Atlantic went on into 1943, the American and British Allies benefited from new inventions which helped them to find and destroy U-boats. For example, High Frequency Direction Finding (known as Huff Duff) could detect even very short radio signals from a surfaced U-boat, allowing convoys to be directed away from the area. Of special help to the Allies was the discovery of an 'Enigma' coding machine on board a captured U-boat. This allowed British intelligence workers to decipher the secret Ultra codes which the Germans used for sending orders to the U-boats.

Despite new weapons and inventions, the Allies were losing the Battle of the Atlantic by early 1943. Their worst losses came in March 1943 when forty U-boats attacked two Allied convoys in the 'Black Gap', an area in the central Atlantic beyond the range of aircraft cover. Out of ninety-two ships in the convoys, twenty-ones were sunk for the loss of only one U-boat.

Encouraged by their success the Germans massed sixty U-boats for an attack on a slow convoy in May 1943. This time, however, the Allies were better prepared. Using a combination of new, fast escort boats, radar, Huff Duff, and long-range aircraft, they sank seven U-boats for the loss of twelve ships in the convoy. In all of that month, the Allies destroyed forty-one U-boats.

Although the U-boats continued to hunt Allied convoys for the rest of the war, the May 1943 sinkings were a turning point in the Battle of the Atlantic. The Germans had failed to cut Britain's supply lifelines across the ocean. With American aid, Britain would be able to continue fighting for as long as it took to defeat Germany.