Germany and Japan
The Americas.
Democrat
Germany.
Germany was Britain's main enemy.
Japan
The question is unanswerable as asked since it does not define who the "enemy" is.
During WW 1, submarines played a major part of a major war for the first time. The role of a submarine was basically to use torpedoes to sink enemy warships and cargo vessels.
Mainly u-boats, other enemy ships and enemy bombers
ASDIC
Japan
Russia.