IIRC, after Almus sets up some stone fortresses at his border...
Some serfs from Diabelwhatever’s towns desert because of famine or unfair taxation. It’s supposed to be a death penalty if they flee, and to “harass” or fuck with Almus’s men on guard, they butcher the peasants... until they can’t take it anymore and intervene, causing a reason for Diabelwhatever to attack Almus.
Also salt mines...?
Or not. I think the LN/Manga is very different from the WN I read, and my memory’s bad and I could be thinking of a different, yet similar isekai since a lot of isekai have a ton of overlap.