How did submarine torpedoes affect WW 2?

The torpedo was the number one killer of ships and submarines, on all sides, during WW2. The total tonnage of ships sunk by torpedo vastly outweighs that of the second-most-common weapon, the aerial bomb, with the naval mine and naval gunfire contributing a miniscule amount.

From the Allied side, torpedoes were useful again the German U-Boats (though bombs, depth charges, the "Hedgehog" device, and other weapons counted for more UBoat kills) and an important weapon in the Battle for the Atlantic, primarily against German coastal shipping and many German surface ships engaged in shipping raiding.

In the Pacific and Mediterranean theaters, Allied torpedoes were devastating against Axis shipping, claiming over 90% of all tonnage sunk during the war. They are also likely the number one killer of Axis naval vessels, though aerial bombing may vie for that title. Torpedoes resulted in the almost complete starvation (of both food and war material) of Axis forces in North Africa andthroughout the entire Japanese Empire.

For the Axis, the torpedo was the U-Boat's primary weapon, and it was extremely effective. Virtually all Allied shipping losses (and the majority of naval warship losses) in the Battle for the Atlantic were due to submarine-launched torpedoes. The U-Boat-launched torpedo was so spectacularly successful when first deployed that Great Britain very nearly collapsed due to an inability to import sufficient food and war material - exactly how close we may never know, but in the dark days of 1940, U-Boats were sinking ships faster than either the US or UK could build them, and the UK was importing less than half its required monthly food.

The German Uboats and Italian motor-torpedo boats also were highly successful in the Mediterranean theater, accounting for the majority of allied shipping and naval losses, though it was closely followed by aerial bombs.

In the Pacific and Indian theaters, Japanese submarines scored significant successes, but the real threat was aerial torpedoes, launched from planes. Japanese aerial torpedoes account for the majority of ships sunk by planes. Additionally, the Japanese "Long-Lance" torpedo - mostly mounted on destroyers - was highly effective against Allied naval vessels, sinking more Allied ships than any other source in 1941-42.

Frankly, it is hard to think of any other weapon, land or naval, that had a greater impact on the course of WW2.