The Battle of the Atlantic, a nautical campaign which lasted the duration of the war, started after the German invasion of Poland with the torpedoing of the British liner SS Athenia by a German submarine (U-boat). Having faced raids on shipping during the First World War, the British quickly implemented a convoy solution to protect merchant vessels; they were short of escort ships though, so many merchant ships had to sail without protection. At first, U-boats primarily operated within British waters while the Atlantic Ocean was covered by German surface vessels. The British attempted to counter the U-boat threat by forming anti-submarine hunting groups, which were ultimately ineffective because the U-boats proved too elusive. With the German conquest of Norway and France by June 1940, U-boats enjoyed decreased resistance. The French Navy was removed as an Allied force, and additional ports in France on the Atlantic Ocean became available to the German Navy (Kriegsmarine), allowing them to increase the range of their vessels. The Royal Navy became severely stretched, having to remain stationed in the English Channel to protect against a German invasion, send forces to the Mediterranean Sea to make up for the loss of the French fleet, and provide escort for merchant vessels. This was somewhat mitigated by the Destroyers for Bases Agreement with the United States Navy in September 1940, in which the British exchanged several of their oversea bases for fifty destroyers which were then used for escort duties. The success of U-boats in this period led to an increase of their production and the development of the wolf pack technique. The German surface navy, which had suffered substantial losses in the capture of Norway, had mixed results. While there were several successful merchant raids, such as Operation Berlin, they also suffered several losses, such as the heavy cruiser Graf Spee and battleship Bismarck. The loss of the Bismarck had deeper ramifications on naval policy though, because as a result Hitler ordered all heavy surface vessels to Norwegian waters[2], shifting them from raiding operations to protection from a potential Allied invasion of Scandinavia. While the Royal Navy also suffered the loss of capital ships, such as the aircraft carrier HMS Courageous, the battleship HMS Royal Oak and the battlecruiser HMS Hood, their larger surface navy was better able to absorb the losses. An Allied tanker torpedoed in the Atlantic in 1942. An Allied tanker torpedoed in the Atlantic in 1942. In May 1941, the British captured an intact Enigma machine, which greatly assisted in breaking German codes and allowed for plotting convoy routes which evaded U-boat positions. In the summer of 1941, the Soviet Union entered the war on the side of the Allies, but they lost much of their equipment and manufacturing base in the first few weeks following the German invasion. The Western Allies attempted to remedy this by sending Arctic convoys, which faced constant harassment from German forces. In September, many of the U-boats operating in the Atlantic were ordered to the Mediterranean to block British supply routes. When the United States entered the war that December, they did not take precautionary anti-submarine measures; this resulted in shipping losses so great that the Germans referred to it as a second happy time. A German U-boat under attack by Allied aircraft in 1943. A German U-boat under attack by Allied aircraft in 1943. In February 1942, several German capital ships that were stationed in the port of Brest, France, managed to comply with Hitler's earlier order and slipped through the English Channel to their home bases in German waters, dealing a significant blow to the Royal Navy's reputation. In June, the Leigh light allowed Allied aircraft to illuminate U-boats that had been detected by the airplanes radar, but this was soon negated by the Germans with Metox, a radar detection system that gave them advance notice of such an aircraft's approach. In American waters, the institution of shore blackouts and an interlocking convoy system resulted in a drop in attacks, and the U-boats shifted their operations back to the mid-Atlantic by August. In December, a strong German surface navy force engaged an Arctic convoy destined for the Soviet Union and failed to destroy a single merchant ship; this resulted in the resignation of Grand Admiral (Großadmiral) Erich Raeder, supreme commander of the Kriegsmarine. He was replaced by Commander of Submarines Karl Dönitz, and all naval building priorities turned to the U-boats. In January, 1943, the British developed the H2S radar system which was undetectable by Metox. As before, this was followed by a counter-invention on the German side, the Naxos radar detector, which allowed German fighters to hone in on Allied aircraft utilizing the H2S. In the spring, the Battle of the Atlantic began to turn in favour of the Allies with the pivotal point being Black May, a period where the Allies had fewer ships sunk and the Kriegsmarine lost 25% of their active U-boats. That December, the German surface fleet lost their last active battlecruiser in the Battle of North Cape. By this time, the Kriegsmarine was unable to regain the initiative; Allied production, such as the mass-produced Liberty ships, improved antisubmarine warfare tactics, sea route patrols with long range attack aircraft, and ever-improving technology led to increasing U-boat losses and more supplies getting through. This allowed for the massive supply build up in the United Kingdom needed for the eventual invasion of Western Europe in mid-1944.
