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The Battle of the Atlantic was the longest continuous military campaign^[ofWorld War II – though some consider it a series of naval campaigns andoffensives– running from 1939 through to the defeat of Germany in 1945. It was at its height from mid-1940 through to the end of 1943. The Battle of the Atlantic pittedU-boats and other warships of the German Navy (Kriegsmarine) against Alliedconvoys. The convoys of merchant ships, coming mainly from North America and the South Atlantic and 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 from September 13, 1941.^[6] The Germans were joined by submarines of the Italian Royal Navy (Regia Marina) after Italy entered the war on June 10, 1940.

As an island nation, the United Kingdom was highly dependent on imported goods. Britain required more than a million tons of imported material per week in order to be able to survive and fight. In essence, the Battle of the Atlantic was a tonnage war: the Allied struggle to supply Britain, and the Axis struggle to cut off the merchant shipping which enabled Britain to keep fighting. From 1942 onwards, the Germans also sought to prevent the build-up of Allied supplies and equipment in the British Isles in preparation for the invasion of occupied Europe. The defeat of the U-boat threat was a pre-requisite for pushing back the Germans. The outcome of the battle was a strategic victory for the Allies—the German blockade failed—but at great cost: 3500 merchant ships and 175 warships were sunk for the loss of 783 U-boats.

The name "Battle of the Atlantic", coined by Winston Churchill in 1941, covers a campaign that began on the first day of the European war and lasted for six years, involved thousands of ships and stretched over hundreds of miles of the vast ocean and seas in a succession of more than 100 convoy battles and perhaps 1,000 single-ship encounters. Tactical advantage switched back and forth over the six years as new weapons, tactics and counter-measures were developed by both sides. The British and their allies gradually gained the upper hand, driving the German surface raiders from the ocean by the end of 1942 and decisively defeating the U-boats in a series of convoy battles between March and May 1943. New German submarines arrived in 1945, but they were too late to affect the course of the war.                        

]Early skirmishes (September 1939 – May 1940)

In 1939, the Kriegsmarine lacked the strength to challenge the combined British Royal Navy and French Navy (Marine Nationale) forcommand 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, including most of the available U-boats and the 'pocket battleships' (or Panzerschiff) Deutschland and the Admiral Graf Spee which had sailed out into the Atlantic in August. These ships began an immediate assault on British and French shipping. U-30 sank the 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, and many of the 57 available U-boats were the small and short-range Type II U-boats which were useful primarily for mine-laying 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, although this had 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.

Some British naval officers, and 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. But this strategy was deeply flawed because a U-boat, with its tiny silhouette, was always 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.^{[citation needed]} 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 September 14, 1939, Britain's most modern carrier, HMS Ark Royal, narrowly avoided being sunk when three torpedoes from U-39 exploded prematurely. U-39 was promptly sunk by the escorting destroyers, becoming the first U-boat loss of the war. Failing to learn the lesson, another carrier, HMS Courageous, was sunk three days later by U-29.

Escort destroyers hunting for U-boats continued to be a prominent, but misguided, feature of British anti-submarine strategy for the first year of the war. The U-boats nearly always proved elusive, and the convoys, denuded of cover, were put at even greater risk.

German success in sinking the 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. Prien immediately became a war hero in Germany.

In the South Atlantic, British forces were stretched by the cruise of the "Pocket Battleship" Admiral Graf Spee, which sank nine merchant ships of 50,000 tons in the South Atlantic and IndianOceans during the first three months of war. The British and French formed a series of hunting groups including 3 battlecruisers, 3 aircraft carriers and 15 cruisers to seek the raider and her sister Deutschland which was operating in the North Atlantic. These hunting groups scoured the oceans for months with no success until the Graf Spee was caught off the mouth of the River Plate by an inferior British force. After suffering damage in the subsequent action, she took shelter in neutral Montevideo harbour and the ship was soon scuttled in December 1939.

After an initial burst of activity, the Atlantic campaign quieted down. Karl Dönitz 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 operations could not be sustained because the boats needed to return to harbour to refuel, re-stock and refit. The harsh winter of 1939–40, which froze over many of the Baltic ports, seriously hampered the German offensive by trapping several new U-boats in the ice. Finally, Hitler's plans to invade Norway and Denmark in the spring of 1940 led to the withdrawal of the fleet's surface warships and most of the ocean-going U-boats to prepare for fleet operations in Operation Weserübung.

The resulting Norwegian campaign revealed serious flaws in the magnetic influence pistol of the U-boats' principal weapon, the torpedo. Although the narrow fjords gave U-boats little room for maneuver, the concentration of British warships, troopships and supply ships provided countless opportunities for the U-boats to attack. Time and again, U-boat captains tracked British targets and fired only to watch the ships sail on unharmed as the torpedoes exploded prematurely or not at all, or ran straight underneath the target. Not a single British warship was sunk by a U-boat in more than 20 attacks. As the news spread through the U-boat fleet, it began to undermine morale. The director in charge of torpedo development continued to claim it was the crews' fault. In early 1942 the problems were determined to be magnetic problems from the high latitude and a slow leakage of high-pressure air from the submarine into the torpedo's depth regulation gear. Eventually theKriegsmarine copied some captured British torpedoes which were much more reliable.

On September 1, 1939, the day Germany attacked Poland, then Commodore Karl Dönitz, Leader U-Boats, submitted to Admiral Erich Raeder a memorandum on his vision of a future war at sea: 300 U-boats could bring Britain to her knees by way of total war against the island nation's vital overseas commerce.^[7]

Dönitz had advocated a system known as the Rudeltaktik or wolf pack, in which groups of U-boats would attack en-mass in mid-ocean and overwhelm any defending warships. While the warships engaged in a cat-and-mouse hunt with individual submarines, the rest of the submarines in the wolf pack would be unmolested, and able to attack the merchant shipping with impunity. In order to be effective, Dönitz calculated that he would need 300 of the latest Atlantic Boats (the Type VII), which would create enough havoc among British shipping that Britain would be knocked out of the war.

This was in stark contrast to the traditional view of submarine deployment up until then, in which the submarine was seen as a lone ambusher, waiting outside an enemy port to attack ships entering and leaving. This had been a very successful tactic used by British submarines in the Baltic and Bosporusduring World War I, but it could not be successful if port approaches were well patrolled. There had also been naval theorists who held that the submarine should be attached to a main fleet and used in a similar way to a destroyer—this had been tried by the Germans at Jutland with poor results since underwater communications were in their infancy. The Japanese also adhered to the idea of a fleet submarine and never used their submarines either as port blockaders or for convoy interdiction. However, the submarine was still looked upon by much of the naval world as a "dishonorable" weapon, compared to the prestige attached to capital ships. This was true in the Kriegsmarine as well, and the Grand Admiral, Erich Raeder, successfully lobbied for the money to be spent on capital ships instead.

The Royal Navy's main anti-submarine weapon before the war was the inshore patrol craft, armed with hydrophones, a small gun and depth charges. The British Royal Navy, like most navies, had not considered anti-submarine warfare as a tactical subject during the 1920s and 1930s. Unrestricted submarine warfare had been outlawed by the Treaty of Versailles; anti-submarine warfare was seen as 'defensive' rather than dashing; and many naval officers believed that anti-submarine work was drudgery similar to mine-sweeping. Though fast destroyers also carried depth charges, it was expected that these ships would be used in fleet actions rather than coastal patrol, so they were not extensively trained in their use.

[edit]'Happy Time' (June 1940 – February 1941)

The German occupation of Norway in April 1940, the rapid conquest of the Low Countries and France in May and June and the Italian entry into the war on the Axis side in June transformed the war at sea in general and the Atlantic campaign in particular in three main ways:

§                     Britain lost her biggest ally. In 1940, the French Navy was the fourth largest in the world. Only a handful of French ships joined the Free French Forces and fought against Germany, though these were later joined by a few Canadian-built corvettes which played a small but important role in the campaign. With the French fleet removed from the campaign, the Royal Navy was stretched even further. Italy's declaration of war in June meant that Britain also had to reinforce her Mediterranean Fleet and establish a new squadron at Gibraltar, known as Force H, to replace the French fleet in the Western Mediterranean.

§                     The U-boats gained direct access to the Atlantic. Since the English Channel was relatively shallow and blockaded with minefields by mid-1940, U-boats were ordered not to traverse it and instead travel around the British Isles to reach the most profitable hunting grounds. The French bases at Brest, Lorient, La Pallice and La Rochelle were about 450 miles (720 km) closer to the Atlantic than the German bases on the North Sea. This greatly extended the range of U-boats in the Atlantic, enabling them to attack convoys further west and letting them spend longer time on patrol, doubling the effective size of the U-boat force. The Germans later built huge fortified concrete bunkers for the U-boats known as U-boat pens in the French Atlantic bases, which were impervious to Allied bombing until Barnes Wallisdeveloped his tallboy bomb. From early July, U-boats began returning to the new French bases when they completed their Atlantic patrols.

§                     British destroyers were diverted from the Atlantic. The Norwegian Campaign and the German invasion of the Low Countries and France imposed a heavy strain on the Royal Navy's destroyer flotillas. The Royal Navy withdrew many of its older destroyers from the convoy routes to support the Norwegian operations in April and May and then diverted them to the English Channel to support the withdrawal from Dunkirk. By the summer of 1940 Britain faced a serious threat of invasion. The destroyers were held in the channel where they would be ready to repel a German invasion fleet. The destroyers suffered heavily in these operations when they were exposed to air attack by theLuftwaffe's Fliegerführer Atlantik. Seven destroyers were lost in the Norwegian campaign, another six at the Battle of Dunkirk and a further 10 in the Channel and North Sea between May and July, many of them to air attack because they lacked an adequate anti-aircraft armament.^[8] Dozens of other destroyers were damaged.

The completion of Hitler's campaign in Western Europe meant that the U-boats that had been withdrawn for the Norwegian campaign were now released from fleet operations and returned to the war on trade. So at the very time that the number of U-boats on patrol in the Atlantic began to increase, the number of escorts available for the convoys, which consisted of between 30 and 70 mostly unarmed merchant ships,^[9] was greatly reduced. The only consolation for the British was that the large merchant fleets of occupied countries like Norway and the Netherlands were under British control. Britain occupied Iceland and the Faeroe Islands to gain bases for themselves and prevent the countries from falling into enemy hands following the German occupation of Denmark and Norway.

It was in these circumstances that Winston Churchill, who had become Prime Minister on May 10, 1940, first wrote to the U.S. PresidentFranklin Roosevelt to request the loan of 50 obsolete U.S. destroyers. This eventually led to the loan (effectively a sale but painted as a loan for political reasons) of the 50 old destroyers under the Destroyers for Bases Agreement in exchange for 99-year leases on certain British bases in Newfoundland, Bermuda and the West Indies, a financially advantageous bargain for the United States, whose population was opposed to entering the war and whose politicians considered that Britain and her allies might actually lose. But the first of these destroyers was only taken over by their British and Canadian crews in September and all needed to be rearmed and fitted with ASDIC. It was to be many months before the relatively obsolete destroyers began to contribute to the campaign.

The early U-boat operations from the French bases were spectacularly successful. This was the heyday of the great U-boat aces like Günther Prien of U-47, Otto Kretschmer of U-99, Joachim Schepke of U-100, Engelbert Endrass of U-46, Viktor Oehrn of U-37 and Heinrich Bleichrodt of U-48. The U-boat crews became heroes at home in Germany. From June until October 1940, over 270 Allied ships were sunk: this period was referred to by U-boat crews as "Die Glückliche Zeit", the Happy Time^[10].

The biggest challenge for the U-boats was to find the convoys in the vastness of the ocean. The Germans had a handful of very long-range Focke-Wulf 200 Condor aircraft based at Bordeaux andStavanger which were used for reconnaissance, but being essentially a converted civilian airliner, this was a stop-gap solution. Due to ongoing friction between the Luftwaffe and Kriegsmarine, the primary source of convoy sightings was the U-boats themselves. Since a submarine's bridge is very close to the water, their range of visual detection was quite limited.

In response to this the British applied the techniques of Operational Research to the problem and came up with series of counter-intuitive solutions to the problem of convoy engagements. It was realised that the area of a convoy increased by the square of its perimeter and that in terms of location a large convoy was as difficult to locate as a small one.

Large convoys with a weak escort (in terms of being more difficult to find) were thus safer than small convoys with a strong escort.

Instead of attacking the Allied convoys singly, the German U-boats were encouraged to work in packs (Rudels) coordinated centrally by radio. German codebreaking efforts at B-Dienst had succeeded in decyphering the British Naval Cypher No. 3, allowing the Germans to estimate where and when convoys could be expected. The boats spread out into a long patrol line that bisected the path of the Allied convoy routes. Once in position, the crew scanned the horizon with binoculars looking for ship's masts or smoke, or used hydrophones to pick up the propeller noises of the convoys. When one boat sighted a convoy, it would report the sighting to U-boat headquarters before tracking it and waiting for other boats to come up, typically at night. Instead of being faced by single submarines, the convoy escorts then had to cope with groups of up to half a dozen U-boats attacking simultaneously. The most daring commanders, like Otto Kretschmer, penetrated the convoy's escort screen and attacked from within the columns of merchantmen in the convoy. The escort vessels, which were too few in number and often lacking in endurance, had no answer to multiple submarines attacking on the surface at night as their ASDIC detection apparatus only worked well against underwater targets. Early British marine radar, working in the metric bands, lacked target discrimination and range.

Pack tactics were first used successfully in September and October 1940, to devastating effect in a series of convoy battles. On September 21, Convoy HX 72 of 42 merchantmen was attacked by a pack of four U-boats, losing eleven ships sunk and two damaged o