At the outbreak of World War I in 1914, submarine technology had evolved to the point that the United States, the United Kingdom, Germany and Russia had all developed diesel-powered submarines that could operate on electrical batteries underwater. The U.S. Navy built 28 subs between 1901 and 1914, increasing its small fleet to 50. World War I introduced the concept of unrestricted submarine warfare, in which not only enemy warships but merchant vessels of all nations found in enemy waters were subject to unannounced attack. The German U-boat service, with an average of only 30 submarines at sea at any one time, put a stranglehold on shipping and merchant supply lines, and nearly brought the United Kingdom to its knees in four years of conflict.
Two decades after the end of World War I, submarines again became important weapons as the world entered World War II. Between the wars, scientists and shipbuilders had improved the basic technology of the diesel-electric submarine. They also had made advances in torpedo design as well as in ways to identify, locate, and destroy enemy subs. When the United States entered the war against Germany and Japan after the December 7, 1941 attack on Pearl Harbor, the U.S. Navy found itself with only one effective weapon in the Pacific theater—its submarine force. On the other side of the North American continent, Germany found the only weapon it possessed that could inflict damage on America was its battle-scarred U-boat force.
This early U-boat shows the small size of these cramped ocean-going submarines. During World War I (1914-1918) and World War II (1939-1945) U-boats lurked underwater and used torpedoes and their deck guns to sink surface ships.
Both navies went to war with limited numbers of subs. The Germans had only 6 operational U-boats out of 51 within reach of the North Atlantic coast at the outbreak of war, while the U.S. Navy at Pearl Harbor had only 14 effective submarines in December 1941. Both sides were also plagued by flawed torpedoes that frequently failed to explode. Nevertheless, both navies scored an immediate impact with submarines. A handful of German U-boats soon wreaked havoc in coastal shipping lanes from Halifax, Nova Scotia, to the Caribbean Sea, while U.S. submarines began long trans-Pacific deployments into Japanese-controlled waters to target both merchant shipping and naval forces. The Allies would go on to win the Battle of the Atlantic against the U-boats in May 1943 through superior antisubmarine warfare technology and the breaking of German naval codes.
The Japanese fought the war with an adequate submarine force of 190 submarines and excellent torpedoes, but failed to capitalize on their potential and ignored the strategic importance of antisubmarine warfare. American shipyards poured over 130 new U.S. submarines into the Pacific fleet during the war. The U.S. submarine force went on to sink more than 1300 Japanese merchant ships, 8 aircraft carriers, 1 battleship, and 11 cruisers—a total of 4.75 million tons of ships, accounting for 55 percent of Japan's shipping losses.