But there was also a fair amount of migration; think of Caesar's Gallic Wars being largely triggered by the intended migration of the Helvetii from the Alps to the Atlantic. Tribal expansions had a knock-on effect of their neighbours seeking to relocate, causing a bit of a cascade. Even the Romans had expeditions where they did not intend to occupy, at least not initially, like an early form of reconnaissance-by-fire. I do think there is a justification for some sort of "passing through" mechanic.
To avoid moving far from historical precedent, the restraints could maybe use a supply mechanic.
e.g. a nomadic tribe would take its "supply centre" with it - a temporary capital which can relocate easily. Armies from settled nations would rely on a supply line from home territory. Breaking that would effectively force a retreat.
If you pass through a territory and want to remain supplied, you have to maintain a controlled line of territories leading to your capital (static or nomadic). However, these could still be regarded as "occupied" as distinct from "owned". An occupied territory would provide no benefit other than pillage/forage and safe supply. This would also allow for enemies to deliberately cut supply lines, similar to how ships can blockade trade links.
