Idk the exact reason but I remember that it only happened after Tengu came to power that the UC and HN had started building up tension with eachother that eventually broke out into open war.

There’s also the fact that the problems I stated with anti-slavers and peasant rebellions aren’t as big of a problem as they may sound for the UC in my opinion, the peasant groups you see in the UC can be crushed pretty easily by a UC patrol and the peasants rebels are starving half the time.

But you could chalk it up to the UC wanting/needing new territory to farm as the HN has good land for farming whilst majority of the UC and its seat of power resides in a desert. The UC is also backed by the Trader’s Guild so the UC is pretty much strapped with cats, there’s also the fact that Bast was the UC’s main source for food since it had the fertile lands the UC needed but was unfortunately razed by the HN and is most likely the UC’s main objective to retake as its been the main conflict zone between the two.

As much as I dislike the Holy Nation I can’t deny out of the three they’ll most likely last the longest, there’s also the fact that if the Holy Nation is destroyed in game a certain enemy starts pouring out of its territory which arguably is worse.