During WW2, all the major countries involved used submarines.
it was Germany.
The US revolutionary war
Enemy surface ships are the main target of submarine warfare
Submarine - A ship designed to travel underwater used for exploration and warfare. Submarine - A sandwich that is shaped like the submarine ship, also known as a grinder or hoagie.
Germany hoped that submarine warfare would lead to war.
Anti submarine warfare
Germany's submarine warfare was unrestricted. These attacks led to World War I.
Principally the Imperial German Navy and the Royal Navy - both enormous forces at the time. Submarines and anti-submarine warfare were still in their infancy in WW1, but it was a German submarine that torpedoed the liner Lusitania off the coast of Ireland.
USA
submarine warfare
Submarine warfare has existed since the early 1900's, and continues to this day. Though the last ship sinking by a submarine was in the Falklands War, the evolving missions of submarines have taken submarine warfare well beyond just sinking ships and submarines.
unrestricted submarine warfare