Unrestricted submarine warfare is a type of naval warfare in which submarines sink vessels such as freighters and tankers without warning. This kind of warfare were designed to impose a naval blockade on countries, especially those heavily dependent on merchant shipping to supply their war industries and feed their populations, when their enemies were unable to institute a conventional naval blockade by surface ships.
Unrestricted submarine warfare in the New World[]
Gra Valkas Empire[]
Gra Valkas Empire is known to have practiced unrestricted submarine warfare from the very beginning of the New World War. The first known account is during the Skirmish at New Japan Sea and successfully during Mu's Eastern Sea Incident when a submarine launched dive bomber attacked a Japanese merchant ship. The submarine used by Gra Valkas when conducting unrestricted submarine warfare is the Cetus-class submarine.
Japan[]
During the World War II, Japan makes an unrestricted Submarine Warfare, raiding Western shipments mainly on the maritime route around Eastern Australia in order to delay the allies efforts in pushing Japan back. However, the Japanese naval doctrine of this period didn't focus on maritime raiding because it was inconsistent with the Kantai Kessen (Decisive Battle) doctrine. Plus with its shortcomings in submarine technology and tactics, Japan has never been able to achieve the same success as Germany in the Atlantic.
In the New World War, as part of the plan to liberate Leifor and the surrounding territories occupied by the Gra Valkas Empire, Japan is cooperating with Mu and Holy Milishial Empire to hold a maritime blockade campaign to the West of the Second Civilization Area. The Assault on Irnetia Kingdom can be considered as the prelude of this campaign.