Britain needed a million import tons a week to stay in the war
The Battle of the Atlantic ran from 1939 to Germany’s 1945 defeat—the longest continuous WWII campaign. Allied blockade met U-boat counterblockade as convoys from North America fed Britain and the Soviet Union across millions of square miles of contested ocean.
No campaign of the Second World War lasted longer. It opened with the Allied naval blockade of Germany, declared the day after war began in 1939, and Germany's answering counterblockade, and it continued until the German surrender in May 1945, peaking between mid-1940 and the end of 1943. Across millions of square miles of ocean, U-boats, surface warships and Luftwaffe aircraft hunted convoys sailing mainly from North America to Britain and the Soviet Union. British and Canadian navies and air forces did most of the escorting, joined by American ships and planes from 13 September 1941; Italian submarines sided with Germany after Italy entered the war on 10 June 1940.
At heart it was a tonnage war. Britain, an island, needed over a million tons of imports every week to keep living and fighting, so the Axis tried to sink merchant ships faster than they could be replaced, while rationing at home cut demand. From 1942 Germany also aimed to stop the Allies stockpiling men and equipment in Britain for the invasion of occupied Europe. Churchill later admitted that the U-boat threat was the only thing that truly frightened him, worrying him more than the Battle of Britain.
The roots lay in the previous war. Attempts to ban submarines after 1918 failed, and the London Naval Treaty instead imposed cruiser rules requiring a submarine to surface and see crews to safety before sinking a ship. Versailles had barred Germany from U-boats, but the Anglo-German Naval Agreement of 1935 let Hitler build a fleet 35 percent the size of Britain's. U-boat chief Karl Donitz calculated that 300 Type VII boats could sink a million tons a month and strangle Britain's roughly 3,000 merchant ships, beating convoys with wolfpack tactics.
The Allies gradually took the upper hand, overcoming German surface raiders by late 1942 and the U-boats by mid-1943, though sinkings continued to the end. The price was 3,500 merchant ships and 175 warships, against 783 U-boats and 47 German surface warships, among them the battleships Bismarck, Scharnhorst, Gneisenau and Tirpitz. Germany spent more on warships than on all ground vehicles, tanks included. The label caught on after 5 March 1941, when A. V. Alexander, then First Lord of the Admiralty, pressed Parliament for extra ships and sailors.
Source: Battle of the Atlantic
