Unrestricted submarine warfare
Unrestricted submarine warfare is a type of naval warfare in which submarines sink merchant ships such as freighters and tankers without warning. The use of unrestricted submarine warfare has had significant impacts on international relations in regards to both the First World War and the Second World War. Its history has been dominated by German decision making.
There have been attempts to limit the use of unrestricted naval warfare, with some dating back to before the turn of the 20th Century as an extension of rules for surface raiders. While initially submarines operated successfully by attacking on the surface using deck guns, attacking without warning while submerged reduces the opportunity for the target to escape or defend itself if armed.