The Battle of the Atlantic, the longest continuous military campaign in World War II, ran from 1939 to the defeat of Nazi Germany in 1945, covering a major part of the naval history of World War II. At its core was the Allied naval blockade of Germany, announced the day after the declaration of war, and Germany's subsequent counterblockade. The campaign peaked from mid-1940 to the end of 1943.
The Battle of the Atlantic pitted U-boats and other warships of the German Kriegsmarine (navy) and aircraft of the Luftwaffe (air force) against the Royal Navy, Royal Canadian Navy, United States Navy, and Allied merchant shipping. Convoys, coming mainly from North America and predominantly going to the United Kingdom and the Soviet Union, were protected for the most part by the British and Canadian navies and air forces. These forces were aided by ships and aircraft of the United States beginning on 13 September 1941. The Germans were joined by submarines of the Italian Regia Marina (royal navy) after Germany's Axis ally Italy entered the war on 10 June 1940.
As an island country, the United Kingdom was highly dependent on imported goods. Britain required more than a million tons of imported material per week in order to survive and fight. The Battle of the Atlantic involved a tonnage war: the Allies struggled to supply Britain while the Axis targeted merchant shipping critical to the British war effort. Rationing in the United Kingdom was also used with the aim of reducing demand, by reducing wastage and increasing domestic production and equality of distribution. From 1942 onwards, the Axis also sought to prevent the build-up of Allied supplies and equipment in the UK in preparation for the invasion of occupied Europe. The defeat of the U-boat threat was a prerequisite for pushing back the Axis in western Europe. The outcome of the battle was a strategic victory for the Allies—the German tonnage war failed—but at great cost: 3,500 merchant ships and 175 warships were sunk in the Atlantic for the loss of 783 U-boats and 47 German surface warships, including 4 battleships (Bismarck, Scharnhorst, Gneisenau, and Tirpitz), 9 cruisers, 7 raiders, and 27 destroyers. This front was a main consumer of the German war effort: Germany spent more money to produce naval vessels than every type of ground vehicle combined, including tanks.
The Battle of the Atlantic has been called the "longest, largest, and most complex" naval battle in history. Starting immediately after the European war began, during the Phoney War, the battle lasted for over five years until the German surrender in May 1945. It involved thousands of ships in a theatre covering millions 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, countermeasures and equipment were developed. 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 the war's end. British prime minister Winston Churchill later wrote, "The only thing that really frightened me during the war was the U-boat peril. I was even more anxious about this battle than I had been about the glorious air fight called the 'Battle of Britain'."
Name
On 5 March 1941, the First Lord of the Admiralty, A. V. Alexander, asked Parliament for "many more ships and great numbers of men" to fight "the Battle of the Atlantic", which he compared to the Battle of France fought the previous summer. The first meeting of the Cabinet's "Battle of the Atlantic Committee" was on 19 March. Churchill claimed to have coined the phrase "Battle of the Atlantic" shortly before Alexander's speech, but there are several examples of earlier usage.
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 "cruiser rules", which demanded they surface, search and place ship crews in "a place of safety" (for which lifeboats did not qualify, except under particular circumstances) before sinking them, unless the ship in question showed "persistent refusal to stop...or active resistance to visit or search". These regulations did not prohibit arming merchantmen, 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.
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 were due to be replaced, the Germans opted to construct the Deutschland-class of 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.
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. A building programme for four battleships, two aircraft carriers, five heavy cruisers, destroyers and U-boats was immediately initiated. After the agreement, Hitler thought conflict with the UK very unlikely; consequently, the fleet was designed for commerce raiding against the French, not to challenge command of the sea. The commander of the German U-boats, Karl Dönitz, had his own opinions. In contrast with Hitler and Erich Raeder, chief of the German navy, he considered war with the UK inevitable and that a large surface fleet was not needed, believing U-boats could defeat the British. According to his calculations, a fleet of 300 medium 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. In the First World War, U-boats had been defeated mainly by the convoy system, but Dönitz thought this could be overcome with the Rudeltaktik, wherein a patrol line of U-boats would search for a convoy before converging and attacking together at night from the surface. Neither aircraft nor early forms of sonar (called ASDIC by the British) were then considered serious threats. ASDIC could not detect a surfaced submarine and its range was less than that of an electric torpedo, aircraft could not operate at night and, during the day, an alert U-boat could dive before the aircraft attacked. Dönitz failed to persuade Raeder, so, each time that the U-boat fleet was expanded, Raeder chose to build a mix 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.
Anti-submarine warfare
With the introduction of ASDIC, the British Admiralty believed the submarine threat effectively neutralized. Consequently, the number of destroyers and convoy escorts was reduced and the anti-submarine branch seen as third rate. Destroyers were also equipped with ASDIC, but it was expected that these ships would be used in fleet actions rather than anti-submarine warfare, so they were not extensively trained in their use. Trials with ASDIC were usually conducted in ideal conditions and the Admiralty failed to appreciate ASDIC's flaws. Its range was limited, worked poorly at speeds above eight knots, was hampered by rough weather and required a very skilled operator to distinguish echoes from thermoclines, whales, shoals of fish and wrecks. Early versions could not look directly down, so contact would be lost during the final stages of a depth charge attack. The basic set could detect range and bearing, but target depth could only be estimated from the range at which contact was lost.
An escort swept its ASDIC beam in an arc from one side of its course to the other, stopping the transducer every few degrees to send out a signal. On detection of a submarine, the escort would close in at moderate speed and increase its speed to attack. The intention was to pass over the submarine, rolling depth charges from chutes at the stern, while throwers fired further charges to either side, laying a pattern of depth charges. To disable a submarine, a depth charge had to explode within about 20 ft (6.1 m). Since early ASDIC equipment was poor at determining depth, it was usual to vary the depth settings on part of the pattern.
As the threat of war loomed in early 1939, Britain realised it could not rely on the London Naval Treaty, which outlawed unrestricted submarine warfare. The organisational infrastructure for convoys had been maintained since World War I, with a thorough and systematic upgrade in the second half of the 1930s, but not enough escorts were available for convoy escorting, and a crash programme for building Tree-class trawlers, Flower-class corvettes and Hunt-class destroyers was initiated. Merchant ships that were either too fast or too slow for convoys were to be equipped with a self-defence gun against surfaced submarine attacks, thus forcing an attacking U-boat to spend its precious torpedoes. This removed these ships from the protection of the cruiser rules under the prize law.
Despite this lack of readiness, in 1939 the Royal Navy probably had as many ASDIC equipped warships in service as all the other navies of the world combined.
Similarly the role of aircraft had been neglected; the Royal Air Force had organised a Coastal Command to support the Royal Navy, but it possessed insufficient aircraft, had no long-range aircraft nor were aircraft crew trained in anti-submarine warfare. The only weapon against submarines was inadequate bombs. Finally, it was not forgotten that in World War I mines had sunk more U-boats than any other weapon. Plans were drafted for minefields in the Channel and along the east coast in defence of shipping lanes, and also offensive mine barrages on the German U-boat lanes towards the Atlantic Ocean.
September 1939 – May 1940
In 1939, the Kriegsmarine lacked the strength to challenge the combined British Royal Navy and French Navy (Marine Nationale) for command of the sea. Instead, German naval strategy relied on commerce raiding using capital ships, armed merchant cruisers, submarines and aircraft. Many German warships were already at sea when war was declared in September 1939, including most of the available U-boats and the "pocket battleships" Deutschland and Admiral Graf Spee which had sortied into the Atlantic in August. These ships immediately attacked British and French shipping. U-30 sank the ocean liner SS Athenia within hours of the declaration of war — in breach of her orders not to sink passenger ships. The U-boat fleet, which was to dominate so much of the Battle of the Atlantic, was small at the beginning of the war; many of the 57 available U-boats were the small and short-range Type IIs, useful primarily for minelaying and operations in British coastal waters. Much of the early German anti-shipping activity involved minelaying by destroyers, aircraft and U-boats off British ports.
With the outbreak of war, the British and French immediately began a blockade of Germany, with little immediate effect on German industry. The Royal Navy quickly introduced a convoy system for the protection of trade that gradually extended out from the British Isles, eventually reaching as far as Panama, Bombay and Singapore. Convoys allowed the Royal Navy to concentrate its escorts near the one place the U-boats were guaranteed to be found, the convoys. Each convoy consisted of between 30 and 70 mostly unarmed merchant ships.
Some British naval officials, particularly the First Lord of the Admiralty, Winston Churchill, sought a more offensive strategy. The Royal Navy formed anti-submarine hunting groups based on aircraft carriers to patrol the shipping lanes in the Western Approaches and hunt for German U-boats. This strategy was deeply flawed because a U-boat, with its tiny silhouette, was likely to spot the surface warships and submerge long before it was sighted. The carrier aircraft were little help; although they could spot submarines on the surface, at this stage of the war they had no adequate weapons to attack them, and any submarine found by an aircraft was long gone by the time surface warships arrived. The hunting group strategy proved a disaster within days. On 14 September 1939, Britain's most modern carrier, HMS Ark Royal, narrowly avoided being sunk when three torpedoes from U-39 exploded prematurely. U-39 was forced to surface and scuttle by the escorting destroyers, becoming the first U-boat loss of the war. Another carrier, HMS Courageous, was sunk three days later by U-29.
German success in sinking Courageous was surpassed a month later when Günther Prien in U-47 penetrated the British base at Scapa Flow and sank the old battleship HMS Royal Oak at anchor immediately becoming a hero in Germany.
In the South Atlantic, British forces were stretched by the cruise of Admiral Graf Spee, which sank nine merchant ships of 50,000 gross register tons (GRT) in the South Atlantic and Indian Ocean during the first three months of war. The British and French formed hunting groups including three battlecruisers, three aircraft carriers, and 15 cruisers to seek the raider and her sister Deutschland, which was operating in the North Atlantic. These hunting groups had no success until Admiral Graf Spee was caught off the mouth of the River Plate between Argentina and Uruguay by an inferior British force. After suffering damage in the subsequent action, she took shelter in neutral Montevideo harbour and was scuttled on 17 December 1939.
After this initial burst of activity, the Atlantic campaign quietened. Admiral Karl Dönitz, commander of the U-boat fleet, had planned a maximum submarine effort for the first month of the war, with almost all the available U-boats out on patrol in September. That level of deployment could not be sustained; the boats needed to return to harbour to refuel, re-arm, re-stock supplies, and refit. The harsh winter of 1939–40 froze many of the Baltic ports, seriously hampering the German offensive by trapping several new U-boats in the ice. Hitler's plans to invade Norway and Denmark in early 1940 led to the withdrawal of the fleet's surface warships and most of the ocean-going U-boats for fleet operations in Operation Weserübung.