When nature ravaged them, they were forced to seek developed resources elsewhere and this’d who had those resources normally would not want to share them with others as all resources were meager and most people lived on the edge of existence where one bad summer meant winter was bare and people starved or died by disease taking advantage of weakened systems.

Thus people had to fight of they’d die themselves. So it wasnt always, let’s make war so we can subjugate more; though that did happen too Vikings. Huns and Ottomans among others.

Of course there were petty rivalries that also just produced unproductive wars.