The German word Unterseeboot simply means submarine, but “U-boat” became closely associated with Germany's campaigns against Allied shipping in both world wars. Their strategic purpose was straightforward: attack the maritime supply system on which Britain's war effort depended.
World War I
Germany initially operated U-boats under restrictions intended to follow established rules for attacks on merchant shipping. Those restrictions became increasingly difficult to reconcile with submarine vulnerability, leading Germany toward unrestricted submarine warfare.
The policy had major strategic consequences. U-boats sank enormous quantities of Allied and neutral shipping, but attacks on passenger and neutral vessels contributed to international opposition and helped shape the path toward American entry into the war.
The Battle of the Atlantic
Germany again turned to submarines during World War II. Type VII boats became the workhorses of the U-boat fleet, operating in groups against Atlantic convoys while attempting to sever the flow of troops, fuel, food and equipment to Britain.
The contest became a technological and intelligence struggle. Allied escort carriers, long-range patrol aircraft, radar, improved sonar, direction finding, codebreaking and more effective convoy tactics steadily reduced the U-boats' advantage.
By 1943, losses had become severe enough to mark a decisive shift in the Atlantic campaign. The U-boat force remained dangerous through the end of the war, but at extraordinary cost to its crews.
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