Hitler had fostered terror over the land, but he also wreaked havoc upon the sea. His U-Boats assaulted any sea going vessel and destroy any military ship they can from the allied side. Admiral Karl Doenitz was the high commander of the German navy’s submarine service throughout World War II, and also served in the U-Boat campaign during World War I. Doenitz convinced Hitler that U-Boats would play a large role in his conquest of EuropeFrom 1938 to 1940 the German submarines have been a thorn in the side of allied vessels. The German navy used to have very few U-Boats in their fleet, but when Hitler was convinced of their usefulness the number grew rapidly. The submarine forces for Germany reached their peak in the fall of 1940. The Nazi’s called this the "happy time" because their U-Boats sank 56 ships in August, 54 in September , and 63 in October. During the "happy time" each U-Boat sank a monthly average of eight vessels. The U-Boats did not stop with just ships of war though.

From 1938 to 1940 the German submarines have been a thorn in the side of allied vessels. The German navy used to have very few U-Boats in their fleet, but when Hitler was convinced of their usefulness the number grew rapidly. The submarine forces for Germany reached their peak in the fall of 1940. The Nazi’s called this the "happy time" because their U-Boats sank 56 ships in August, 54 in September , and 63 in October. During the "happy time" each U-Boat sank a monthly average of eight vessels. The U-Boats did not stop with just ships of war though.

In 1941, Britain had to give up bringing supplies into their southwestern ports because I was too close to the U-Boats home base. To solve their problem with the U-Boats the British started having convoys accompanied by battleships. The ships stood more of a chance in large groups, but still many were sunk. The battleships in the British navy would use sonar to detect the submerged vessels, unfortunately it was not always reliable. Another dilemma was that the sonar only detects submerged vessels, so the submarines would attack from the surface use deck guns and blow holes in the sides of the ships.

To counter the use of convoys, Doenitz developed the "wolf pack". This is a tactical array of U-Boats in groups themselves. The group would trail a convoy and sink the ships. Many ships were lost due to Doenitz commanding of the German submarines. They played a major role in cutting off allied supplies and destroying allied ships.