You can't really compare premodern and modern societies in this regard. Totally different contexts.

In ancient Europe, if you had some lucrative territory and not enough military force to defend it, you would be raided and invaded by your neighbours with absolute certainty. Maybe not the same summer, but quite soon. People had different understanding of morality and wars of conquest were glorified. Plus, quite a lot of the pre-state societies didn't have any centralization of power and a hot-headed noble could absolutely start a war just with his retinue, dragging his entire tribe into it.

This is just not the model on which the current EU works, although the Russo-Ukrainian war was a rude awakening in this regard.