From the opening days of the war, the German Kriegsmarine (Navy) sought to isolate Britain, targeting shipments of food, fuel, and other needed war goods from Commonwealth territories to the home islands. The greatest menace to this lifeline was the German U-boat fleet, lurking beneath the waves and hunting their prey with powerful torpedoes that could tear lethal holes in civilian cargo vessels.

The ubiquitous submarines had considerable early success against merchant ships sailing alone and far from the nearest aid. British destroyers were effective escorts, but steep losses in the battles for Norway and France left few available for such duty. The situation worsened after the triumph over France provided the Germans with new bases along the Atlantic coast, allowing U-boats to probe deep into the ocean and search for their defenseless quarry in “wolf packs” spread across the open sea.

The turnaround began in late 1940, when the British acquired fifty World War I-era destroyers from the United States. They were older model ships but fully capable of keeping pace with the slow-moving transports. The British also returned to an effective defensive tactic from the previous war, grouping the transports into large convoys that could be protected with fewer escorts.

In early 1941, the US Congress passed the Lend-Lease Act, authorizing the Roosevelt Administration to furnish needed war materiel to friendly countries without immediate payment. It was a boon to the British, whose treasury was depleted from the war, and opened the floodgates of goods flowing across the Atlantic. Another decisive turn came that year when British cryptographers managed to unlock the German naval codes and gain access to secret communications. The information was invaluable; convoys were steered away from the wolf packs, and U-boats became the hunted, with British naval forces thwarting efforts to refuel and replenish the submarines at sea.

By 1943, newly built escort aircraft carriers were arriving in the Atlantic from American shipyards, providing the convoys with continuous air support for the length of the crossings. New destroyers were also on-hand, armed with advanced radar and sonar systems and deadly anti-submarine weapons. As sinkings among Allied transports began tapering off, losses of U-boats rose sharply, diminishing the threat in the Atlantic.

The Battle of the Atlantic produced steep losses on both sides. Nearly 3,000 Allied merchant and escort ships were torpedoed, with tens of thousands of crewmembers lost at sea. The Germans suffered equally, with their submarine fleet virtually wiped out by the end of the war. 781 U-boats were lost, and of the 40,000 German sailors who served in the fleet, more than 30,000 went down inside their “iron coffins.”