In almost all navies during WWII, submarines were called "boats"...not ships. In Germany, a sub was called an "under-sea-boat", with German enthusiasts preferring to use the German spelling/pronunciation of "boot" for boat. Bottom line: "U-Boat submarine warfare..." is simply submarine warfare (Sub Warfare is what most boys in America grew up calling it).
The Uboat was a submarine made by the Germans
It was the German way of saying "submarine" which properly should be called a "submersible." It is a shortening of "unterseeboot" to just uboat.
In World WarII, the germans named it a uboat
Enemy surface ships are the main target of submarine warfare
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.
submarine warfare
USA
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
Germany's unrestricted submarine warfare