The US revolutionary war
Germany hoped that submarine warfare would lead to war.
Germany's submarine warfare was unrestricted. These attacks led to World War I.
unrestricted submarine warfare
Germany's unrestricted submarine warfare
The "Turtle" in the Revolutionary War.
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.
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.
Railroad transport and communicating by telegraph, I guess. Ah yes, submarine warfare.
the Turtle in the Civil War.
Diesel-Electric submarines had been in use for years before WWI; there were many commissioned boats on both sides of the war before it even started, so there is no one particular submarine used in WWI. However, the one boat that arguably had the most impact at the beginning of WWI was the German submarine U-20, which was responsible for sinking the British passenger liner RMS Lusitania. This single submarine attack had repercussions throughout the war, causing policy shifts in Germany considering submarine warfare, and which affected submarine warfare in general forever.
unrestricted submarine warfare
Unrestricted submarine warfare is a naval war tactic in which the submarine attacks merchant ship (or any ship not in the navy) without warning. Germany used this tactic on American vessels at the start of WW1, and that is a major reason that the US joined the war effort against the Germans.