During World War II, the Wehrmacht used some technology that had been developing since before World War I - the U-Boat - a submarine type vessel whose name stands for the German term Unterseeboot meaning Underwater Boat. The U-boat arm of the Kriegsmarine was thus known as Unterseebootwaffe.

Germany's first high seas U-boat, U-36, gave the Kriegsmarine the advantage in the Atlantic and other bodies of water around the world. From 1936 right up until the surrender of Germany in 1945, U-boats sailed the high seas and crippled Allied shipping routes and convoys.

Different classes of U-boats were designed and commissioned throughout the course of the war. Groups of U-boats were called Flotillas, which were organizational command groups.

The main target of the U-boat wars was mostly merchant shipping, especially towards the earlier years of the war. The purpose of attacking merchant ships over warships is that it cripples supply lines and merchant ships are usually poorly armed and armored. When 1940 rolled around there were enough U-boats to travel in their highly effective wolfpacks - or Rudeltaktik. The idea was the brainchild of Großadmiral Karl Dönitz. A single U-boat would chase the convoy, and in the meantime the others would effectively surround the convoy ships. The U-boats would then be able to fire from all directions, confusing and overwhelming any convoy escorts and inflicting as much damage as possible. An average wolfpack contained 3 to 4 U-boats, while some contained almost 20!

U-boats served in theatres all around the world, not just the Atlantic. Some boats saw service in the Mediterranean, Caribbean, Black Sea, and even some out near Asia and Australia!

Some of the more successful U-boats were: U-48 sinking 51 ships and 307,000 tons, U-99 sinking 38 ships worth 244,000 tons, and U-103 dropping 45 ships at 238,000 tons total. There were approximately 1154 U-boats commissioned during the time of World War II.

During the second half of 1942, the improvement of Allied anti-submarine measures began to turn the tide. U-boats started to be sent on patrols alone, rather than in wolfpacks to minimize losses if they were found. There were even a deal of U-boats that ended up being sunk themselves before damaging any foreign vessels! Allied aircraft support and firepower became more prevalent around convoys and escorts. The allies also got better technology at depth charging and finding U-boats with sonar technology; as well as the fact that the British admirality had cracked the Enigma code - used by BdU (Befehlshaber der U-boote, or the U-Boat High Command) to send orders and information to uboats at sea... without the German's knowing their code was compromised.

In the first 6 months of 1942, only 21 U-boats had been lost, but in the second half that number rose to 64. In the first months of the following year, 94 boats were sunk (41 in May alone)! After "Black May", as it became called, the number of ships sunk by U-boats dropped significantly. There was a ratio of 13.58 merchant ships per U-boat lost in 1942. In 1943 that figure change to only 1.89 ships per U-boat sunk. This brought BdU to admit to defeat. Großadmiral Karl Dönitz ordered the withdrawl of U-boats from the North Atlantic on 24 May, 1943.

Operation Rainbow, or Regenbogen, was devised by the BdU to preserve what honor was left for the Kriegsmarine. U-boat commanders were to ready their U-boat for scuttling. However, on the 4th of May 1945, Dönitz was forced to retract the order because of conditions from Allied commanders. In spite of this, many U-boat commanders saw this as being against the Admiral's will and scuttled their boats anyway. 231 U-boats were scuttled before the Allies could get to them.

The U-boat war was a very significant part of Germany's battle against the Allies - helping to control shipping lines and war supplies. Had better U-boat technology emerged earlier in the war, the entire tide of the war may have been effected.

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