Well, close, but Poland was invaded by a number of neighbors (we also did some invading ourselves, back when we actually could).
Your reasoning makes me think of the prisoner's dilemma. Sure, if everyone agreed to just abandon the concepts of cultural identity (leading to nationality) and live in peace, the world would be a better place. But for the moment, I'm afraid that "world peace" is something that only Miss World candidates speak of regularly.
Btw, I'm not suggesting to abandon cultural identity. Those are natural in a sense, while I feel that nationality is a tool to herd people in. Conflicts will continue to happen, no doubt, we're still human. But at least they are somewhat justified clashes, instead of made up wars that serve only the interests of an elite.
Your reasoning makes me think of the prisoner's dilemma. Sure, if everyone agreed to just abandon the concepts of cultural identity (leading to nationality) and live in peace, the world would be a better place. But for the moment, I'm afraid that "world peace" is something that only Miss World candidates speak of regularly.