You make an important point here. “Countries” are mental formations and always have been. That is, they have no reality at all outside our own tribal readings of history. But after WW II, the UN was founded to confer legitimacy upon “nations” based on negotiated borders and the consensus of the inhabitants and their leaders, and the approval of the international community. NOT by brute conquest. As European colonization met indigenous resistance after the war, new “nations” were established all over the world by this consensual process, and likewise when the Soviet empire collapsed. So there is nothing Platonic or “ancient” about nations; they arise through historical processes and are validated by international consensus. And this world order,however imperfect, has largely kept the peace and enabled international trade and diplomacy since 1945. Until Putin abrogated it.