Churchill called U-boats 'the only thing that really frightened me during the war.' For five years, German submarines tried to starve Britain into surrender — and almost succeeded.
The Battle of the Atlantic was the longest continuous military campaign of World War II, running from the first day of the war to the last — September 3, 1939 to May 8, 1945. Britain depended on over a million tons of imported food, fuel, and war material every week to survive. Germany's goal was simple: cut those supply lines with submarines and starve the island nation into surrender.
In 1940 and early 1941, German U-boat commanders called it 'the Happy Time.' Operating in 'wolfpacks,' multiple submarines would coordinate attacks on Allied convoys, picking off merchant ships faster than they could be replaced. U-boat ace Otto Kretschmer sank 44 ships in 18 months. By the end of 1940, Britain was losing shipping faster than it could build replacements.
The tide began to turn through technology and intelligence. Britain cracked the German Enigma cipher machine with help from Bletchley Park, allowing convoys to be routed around wolfpack positions. Shipborne radar let escort vessels detect submarines on the surface. Long-range aircraft closed the 'Black Pit' in the mid-Atlantic where ships had previously sailed without air cover. Each breakthrough cost Germany more submarines and crews.
May 1943 became known as 'Black May' for Germany. In a single month, Allied forces sank 41 U-boats — roughly a quarter of the operational fleet. Admiral Karl Donitz suspended Atlantic operations and wrote in his diary that the navy had 'lost the Battle of the Atlantic.' He was right, though submarines continued to operate and sink ships until the war's end.
The human toll on both sides was staggering. The Allies lost 3,500 merchant vessels and 175 warships, with 36,000 merchant sailors and 36,000 naval personnel killed. Germany lost 783 submarines and approximately 30,000 U-boat sailors — over 75 percent of all men who served in U-boats. It was the highest casualty rate of any branch of any military in the war.
The Battle of the Atlantic shaped the war's outcome in ways that are easy to overlook. Every tank, aircraft, and soldier that reached Britain or the Soviet Union through the Allied supply chain had to survive the U-boat gauntlet first. The Normandy landings, the campaigns in North Africa and Italy, the entire western Allied war effort — all depended on winning a battle fought in darkness and cold on the open ocean.