I think "protect your land or face conquest" is putting it a bit strong. But it's closer to reality than the inverse. Basically, medievals wouldn't have understood the concept of a neutral vs non neutral polity. That took centuries of development in international law. The Treaty of Paris (1815) and the Hague Convention (1907) essentially made the permanent neutrality of Switzerland, Sweden, and some others I'm probably forgetting possible. The closest you could potentially get in the medieval era would be monasteries and church lands, but I'm not comfortable completely separating those from their larger communities.

It's in part because of the absence of any real way to arbitrate disputes between peers - county and county, kingdom and kingdom - that the medieval period was marked by such frequent low-level war. Most medieval wars were neither begun nor prosecuted with complete conquest or destruction in mind. Rather, they fought until one side had taken enough of a beating and knuckled under.