In World War 2, as they did in World War 1, submarines were widely used by both sides as the ultimate weapon of naval blockade, sinking large numbers of both merchant ships and warships, resulting in either paralyzing the enemy's military industry and war effort by causing severe shortages of war materials and products and preventing maritime troop transfers, or forcing the enemy to dedicate enormous resources to anti submarine warfare in order to prevent that from happening, resources which could otherwise be used in the enemy's main war effort.
Either way, the submarine is the classic weapon of the war of attrition, where victory is achieved by the side which inflicts significantly more losses in men and material to the enemy than it suffers, and monthly ship tonnage loss counts reflect success or failure better than battle maps. The submarine's ability to attack almost anywhere and anytime, in total surprise and with devastating results, makes it a highly efficient weapon, in which a small crew of about 40 to 80 sailors can do more damage than 1000 sailors in a battleship, and for a fraction of the cost, and a much larger anti-submarine force is required in order to efficiently fight the submarine.

The same features of the submarine also make it the perfect blockade runner, capable of quietly smuggling secret agents and commandos, or small amounts of high importance cargo, to and from almost any enemy beach or a blockaded harbor, making the submarine one of the main vehicles of intelligence and special forces warfare.

Unlike the post war nuclear submarines, World War 2 submarines had quite limited underwater speed, range, and endurance. They usually sailed on the surface, especially at night, and submerged only when they had to, to avoid being detected and attacked. In the first years of the war, German submarines even made group night attacks while surfaced, as if they were surface torpedo boats. As RADAR and