3. It is a situation where subs can destroy any kind of vessel they want.
Unrestricted submarine warfare is a form of naval warfare where any ship coming into the path of a submarine could be shot at or attacked in the name of the warfare.
Germany used the policy of unrestricted submarine warfare during World War I. They used this method to attack any and all ships leaving or coming to Great Britain. The US fell victim to this type of warfare because they had declared neutrality and were continuing to trade with all European powers. Despite the US's attempts at stopping Germany, they continued the policy through the war.
