It start fat guy about to die be transported to other world. He got transported to a battle field and recruited by the demon army. Thing in this world the maou are transported and the past ones taught the demons/monsters some things necessary for civilization. First one taught them agriculture and the succeeding one taught some aspect. One thing they have not taught is warfare and the mc is a military otaku. I remember his immediate predecessor introduced manga/anime culture.
When your predecessors were so good they advanced the civilization hundreds of years ahead so you have to lower the expectations a bit