The outcome of the battle was a strategic victory for the Allies—the German blockade failed—but at great cost: 3,500 merchant ships and 175 warships were sunk for the loss of 783 U-boats.

The name “Battle of the Atlantic” was coined by Winston Churchill in February 1941. It has been called the “longest, largest, and most complex” naval battle in history. The campaign started immediately after the European war began, during the so-called “Phoney War“, and lasted six years, until the German Surrender in May 1945. It involved thousands of ships in more than 100 convoy battles and perhaps 1,000 single-ship encounters, in a theatre covering thousands of square miles of ocean. The situation changed constantly, with one side or the other gaining advantage, as participating countries surrendered, joined, and even changed sides in the war, and as new weapons, tactics, counter-measures, and equipment were developed by both sides. The Allies gradually gained the upper hand, overcoming German surface raiders by the end of 1942 and defeating the U-boats by mid-1943, though losses due to U-boats continued until war’s end.

Following the use of unrestricted submarine warfare by Germany in the First World War, countries tried to limit, even abolish, submarines. The effort failed. Instead, the London Naval Treaty required submarines to abide by “cruiser rules“, which demanded they surface, search[16] and place ship crews in “a place of safety” (for which lifeboats did not qualify, except under particular circumstances)[17] before sinking them, unless the ship in question showed “persistent refusal to stop…or active resistance to visit or search”.[18] These regulations did not prohibit arming merchantmen,[19] but doing so, or having them report contact with submarines (or raiders), made them de facto naval auxiliaries and removed the protection of the cruiser rules.[20] This made restrictions on submarines effectively moot.