After being summoned to a fantasy world of swords and magic Sebastian is soon on his way to fighting for the empire in the name of justice but wait, is he really fighting for the right side? He is thrust from his idyllic isekai fantasy into the harsh reality of a war-ridden age.

You can find the same story on Royal Road here: https://www.royalroad.com/fiction/73444/summoned-to-another-world-by-the-bad-guys-progression