The Battle of the North Atlantic began on September 3, 1939, with the sinking of the British liner Athenia by the German submarine U-30. At the time, neither side was prepared for what was to follow. Germany had only fifty-six operational submarines of which only twenty-two were suited for service in the Atlantic Ocean. The British did not attach a high priority to planning and equipment for antisubmarine warfare, thinking that they could improvise at the outbreak of hostilities.

The Battle of the North Atlantic began on September 3, 1939, with the sinking of the British liner Athenia by the German submarine U-30. At the time, neither side was prepared for what was to follow. Germany had only fifty-six operational submarines of which only twenty-two were suited for service in the Atlantic Ocean. The British did not attach a high priority to planning and equipment for antisubmarine warfare, thinking that they could improvise at the outbreak of hostilities.

At the outset of the war, the Germans sought to impose a submarine blockade of all British ports. This proved not to be the best strategy. The commander of U-boats, Admiral Karl Dönitz, was soon convinced that operating in coastal waters made the U-boats too vulnerable to land-based aircraft and that they should instead concentrate in the mid-Atlantic, which was out of reach for most aircraft at that time. In this first phase, U-boats operated singly rather than as a group. However, the Allies, including the U.S. Navy under Ernest King, soon organized a convoy system, and the Germans adapted by attacking in groups called wolfpacks.

Dönitz calculated that sinking a monthly average of 800,000 tons of shipping would cripple the British war effort and bring about its capitulation. Without merchant shipping, the British could neither eat, run their industries, nor continue fighting. In the early years of the war, losses of merchant ships exceeded the rate of ship production by a 2–1 margin. It was not until August, 1942, that ship production balanced the losses.

By April, 1943, more than 400 U-boats were in service, and victory in the Atlantic was almost achieved by the Germans. Abruptly, however, the tide of battle turned. The Allies were able to close the gap and provide air cover the entire width of the Atlantic. Improved radar enabled the Allies to pinpoint the location of German submarines after only a brief radio transmission. The United States was constructing merchant ships at a rate far exceeding losses. More than 2,500 of these wartime merchant vessels known as Liberty ships were built in American shipyards.

In May, 1943, the Germans lost thirty-five submarines with only 96,000 tons of shipping to show for it. It was now clear that it was pointless to send submarines into the North Atlantic to attack shipping as the losses would exceed any reasonable return. Dönitz temporarily withdrew U-boats from the North Atlantic and used them in other locations where they would not be so vulnerable. The U-boats were returned to the Atlantic, but their main function became to slow and harass Allied shipping and force the Allies to continue devoting resources to an antisubmarine campaign, thus preventing these resources from being used elsewhere. The end came on May 4, 1945, when Dönitz sent a radio signal to all U-boats to stop all hostile action against Allied shipping.

The battle was costly to both sides. During the course of the war, the Allies lost more than 2,600 ships. More than 30,000 merchant seaman and supporting naval personnel lost their lives. The Germans lost more than 750 U-boats, which amounted to more than 85 percent of all operational boats. Of the approximate 40,000 who served as U-boat crewmen, 27,491 died in action.

Winning the Battle of the North Atlantic was essential for an Allied victory. Britain could not be sustained nor could offensive operations be undertaken on the European continent without control of the North Atlantic.