Entering the Imperial German Navy in 1911, Karl Dönitz, a Berlin native, served as a submarine commander in WWI as Hitler was coming into power. While underway on deployment in 1918, his UB-68 was badly damaged by British forces and eventually sunk, but Dönitz was captured and transported to a POW camp.
After nine months of captivity, Dönitz was released from custody back to German hands where he was appointed by General Admiral Erich Raeder to command and create a new German U-boat fleet.
Under his new position, he developed a U-boat patrolling strategy called "wolfpack" formations — meaning groups of submarines would maneuver in straight lines. Once the patrolling U-boats came in contact with enemy vessels, they would signal the wolfpack who would then charge forward and attack.
Admiral Karl Dönitz meets with Adolf Hitler (Source: Wikipedia Commons)
In 1943, Dönitz replaced General Admiral Raecher as Commander-in-Chief (the same man who originally assigned him) and with his naval warfare expertise began winning Hitler's trust.
During his role as Commander-in-Chief, Dönitz was credited with sinking nearly 15 million tons of enemy shipping, coordinating reconnaissance missions, and allowing his U-boat commanders to strike at will when they believed they could inflict the most damage. At that same time, he commanded of 212 U-boats and had another 181 on standby used for training. His tactics proved to be superior to those of his enemys until the invention of microwave radar which managed to spot his German created U-boats sooner than before.
After Hitler died, his last will and testament named Dönitz as commander of the armed forces and the new Reich president. For the next 20 days, he served the last leader of Nazi Germany until the British once again captured him on May 23, 1945.
During the Nuremberg Trials, Dönitz was charged with multiple war crimes and sentenced to 10 years in Berlin's Spandau prison. Upon his release, he published two books and continued to state he had no knowledge of any crimes committed by Hitler.
The German admiral died on Christmas Eve, 1980.
Check out the Smithsonian Channel video below to discover some of the German U-boats effects on the war.