Editing Battle of the Atlantic (section)

==Background==
Following the use of [[unrestricted submarine warfare]] by Germany in the [[First World War]], countries tried to limit or abolish submarines. The effort failed. Instead, the [[London Naval Treaty]] required submarines to abide by "[[Prize (law)|cruiser rules]]", which demanded they surface, search{{sfn|Holwitt|2005|pp=5-6}} and place ship crews in "a place of safety" (for which lifeboats did not qualify, except under particular circumstances){{sfn|Holwitt|2005|p=92: quoting Article 22 of the [[London Naval Treaty]].}} before sinking them, unless the ship in question showed "persistent refusal to stop...or active resistance to visit or search".{{sfn|Holwitt|2005|p=93}} These regulations did not prohibit arming merchantmen,{{sfn|Holwitt|2005|p=6}} but doing so, or having them report contact with submarines (or [[Commerce raiding|raiders]]), made them ''de facto'' naval auxiliaries and removed the protection of the cruiser rules.{{Sfn|Blair|1996a|p=65}}{{Failed verification|date=December 2023}}

The [[Treaty of Versailles]] forbade the Germans to operate U-boats and reduced the German surface fleet to a few obsolete ships. When three of these obsolete ships had to be replaced, the Germans opted to construct the [[Deutschland-class cruiser|Deutschland-class]] of {{lang|de|panzerschiffe}} (armoured ships) or  "pocket battleships" as they were nicknamed by foreign navies. These ships were designed for commerce raiding on distant seas, to operate as a raider hunting for independently sailing ships, and to avoid combat with superior forces.{{Sfn|Blair|1996a|pp=22-32}}{{Failed verification|date=December 2023}}

The [[Anglo-German Naval Agreement]] of 1935 allowed Hitler to renounce the treaty of Versailles, and to build a fleet 35% the size of Britain's fleet. A building programme for four battleships, two aircraft carriers, five heavy cruisers, destroyers and U-boats was immediately initiated. With the agreement, Hitler thought that conflict with the UK was very unlikely and hence the fleet was designed for commerce raiding against the French rather than to try to challenge command of the sea.{{Sfn|Blair|1996a|pp=33-35}}{{Sfn|Bekker|1971|pp=19-22}} The commander of the German U-boats, Karl Dönitz, had his own opinions. In contrast with Hitler and [[Erich Raeder]], the chief of the German Navy, he judged that war with the UK was inevitable and that a large surface fleet was not needed, but that U-boats could defeat the British. According to his calculations, a fleet of 300 medium [[Type VII U-boat|Type VII]] U-boats could sink a million tons of ships a month and within a year sink enough of the about 3,000 British merchant ships (comprising 17.5 million tons) to strangle the British economy.{{Sfn|Blair|1996a|pp=37-39}}<ref>Karl Dönitz: ''Gedanken über den Aufbau der U-Bootswaffe, 1. September 1939''. (Bundesarchiv-Militärarchiv, Freiburg, Germany, Case 378, PG 32419a. Seekrieg 1939)</ref>{{sfn|Herwig|2004|p=74}} In the First World War, U-boats had been defeated mainly by the [[convoy]] system, but Dönitz thought this could be overcome with the {{lang|de|[[Wolfpack (naval tactic)|Rudeltaktik]]}}: a patrol line of U-boats searched for a convoy and when one was found all U-boats converged and attacked together at night on the surface.{{sfn|Costello|Hughes|1977|p=30}} Neither aircraft nor early forms of [[sonar]], (called ASDIC by the British), were considered a serious threat at the time. ASDIC could not detect a surfaced submarine and its range was less than that of an [[G7e torpedo|electric torpedo]], aircraft could not operate at night and, during the day, an alert U-boat could dive before the aircraft attacked.{{Sfn|Blair|1996a|p=38}}{{Failed verification|date=December 2023}} Dönitz could not convince Raeder of his ideas, so each time the U-boat fleet was expanded, Raeder opted to build a mixture of coastal, medium and large submarines, even minelayers and U-cruisers. Even when in 1938 Hitler realised he would sooner or later have to oppose the UK and launched his [[Plan Z]], only a minority of the planned 239 U-boats were medium U-boats.{{Sfn|Blair|1996a|pp=45-49}}