Well the topic sums it up but here the full version:

A while back I started reading this manga where there is this History Otaku. One day he gets pulled into a parallel world and then becomes the king of the demons there, who are actually pathetic at fighting (the demons I mean) and are getting creamed by the humans (I think, could be another race or an alliance of races). The History Otaku helps them fight off the humans by teaching them tactics and traps from his knowledge of history. Then they go to the demon capital where they meet the now previous Demon King (who is also a human, I kinda remember him being a slob) who explains that every Demon King brought something for the demons' betterment (culture, skills, etc).

That's all I remember. I really want to read it again and see if there have been any updates but I forgot the name of the manga.

Can anyone please help me?

Is it A-kun (17) no Sensou - I, the Tycoon??

Thanks a lot. I was looking for this one. Now I just gotta hope they release/scanlate more chapters.