Although U-boats had been updated in the interwar years, the major innovation was improved communications and encryption; allowing for mass-attack naval tactics.
By the end of the war, almost 3,000 Allied ships (175 warships, 2,825 merchantmen) had been sunk by U-boats.
The submarine force was the most effective anti-ship weapon in the United States Navy arsenal.
Although constituting only about 2 percent of the U.S. naval force, submarine force destroyed over 30 percent of the Imperial Japanese Navy, and over 60 percent of the Japanese merchant fleet, The Royal Navy Submarine Service was used primarily to blockade trade and military supply routes to Africa and the Near and Far East, but also obtained the only mutually submerged submarine-to-submarine combat kill of World War II.
This occurred when the crew of HMS Venturer engaged the U-864, manually computed a successful firing solution against a three-dimensional moving target using techniques which became the basis of modern torpedo computer targeting systems.