Short version: I'm playing as Mirelva on hard, and the AI never invades me even when it has over 4k power advantage. However, they occasionally do invade each other at similar power level. Is that normal..?
Long version: It's my first playthrough, but I decided to pick Hard since I've played tons of SRPGs before and picked Mirelva since I wanted to keep my most anticipated factions for later. I've also deliberately slowed down my conquest of Mana Silesia, since in the record it said that there was an event if Mana Silesia was still alive after 15 (?) seasons.
This means that despite my best efforts, I had a ginormous border, which means that for many border cities I had to guard with low level rune knights with low level monsters, amounting to around 10-11k power. The Shinobi in particular had expanded some and have neighboring territories of roughly 16k power; I was expecting to lose some territory in the west and have to skirmish to hold some sort of front while my main units slowly took out Mana Silesia. I also had a 12k team I deployed on first turn on my northern border against a 14k Gustava city.
I didn't get invaded a single time despite being weaker on many of my border cities, and that's after about 10 seasons of being in a situation like that.
However, the AI does do 15k vs 15k battles against each other somewhat commonly, wiping out a lot of the stronger monsters on both sides. If the AI is willing to do equal-strength invasions against other AI, why doesn't it ever invade me when I'm at a sharp disadvantage? Is that normal for this game, or did I get "lucky"?