ReMember (KING Gonta)

From MangaHelpers: Finding himself in a war torn Japan that has just lost the war to the US, Zaji wanders the land that used to be a city. Suffering from amnesia, the only thing that he remembers is that he has to remember something important. He quickly learns that he has incredible martial arts combat skills and is able to use it even if he doesn't realize it since it seems that it has been engrained into his body through years of training. He is taken in by Gin, the owner of the local bar that occupying US soldiers frequent. Zaji learns the full extent of his strength as he tries to protect the helpless in a devastated society.