Battle of the Atlantic
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) 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, 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. 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 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, 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 a series of 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 down. 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, which froze over many of the Baltic ports, seriously hampered the German offensive by trapping several new U-boats in the ice. 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 for fleet operations in Operation Weserübung.

The resulting Norwegian campaign revealed serious flaws in the German U-boat torpedoes: both the impact pistol and the magnetic influence pistol (detonation mechanism) were defective, and the torpedoes did not run at the proper depth, often undershooting targets. Only one British warship was sunk by U-boats in more than 38 attacks. As the news spread through the U-boat fleet, it began to undermine morale. Since the effectiviness of the magnetic pistol was already reduced by the degaussing of Allied ships, Dönitz decided to use new contact pistols, which were copied from British torpedoes found in the captured British submarine HMS Seal. The depth setting mechanism was improved but only in January 1942 were the last complications with that mechanism discovered and fixed, making the torpedo a more reliable weapon.
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