In the early days of submarine warfare, there were rules that everyone followed when attacking ships. If a submarine was attacking a ship that wasn't obviously a warship, they were supposed to show themselves to the ship, tell them they were going to sink that ship, and then make sure that the crew and civilians were safely off the ship before it was sunk.
During World War I, the British had such a large navy that the German navy could not defeat it, and the British were able to stop other ships from bringing food and supplies to Germany. Not only that, but the British could have food and supplies sent to England without much fear of the Germans stopping them.
The Germans decided do anything they could to break the British blockade, and to stop food/supply shipments from reaching England. This meant stopping the old rules of using submarines.
However, without the old rules for warning ships that they were going to be sunk and making sure the non-military people were safe, this tactic angered countries who weren't involved in the war but had their ships sunk. The USA was one such country...
submarine
unrestricted submarine warfare
Anti-Submarine Warfare.
To get the blockade lifted.
American enterance into WWI.
submarine
Enemy surface ships are the main target of submarine warfare
Anti submarine warfare
Germany hoped that submarine warfare would lead to war.
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
The Sea people
it was Germany.