Kihei Senki

In an alternate Japan, where the Second World War has suddenly came to an end, a disturbed man returned from the war zone to the imperial capital as it was enjoying a brief period of peace. His name is Kihei Janome, also known as "the demon soldier who killed a thousand people." He escaped being sent to the execution chamber, but when a strange priestess gave him a mysterious command, Kihei was forced to the fierce "front lines" of a new war.

(Source: Shonen Magazine, translated)