I ultimately agree that property rights exist because of societal acceptance that they exist. Others may disagree, but I don't think there are logical principles you can use to objectively say that any particular property claim is legitemate.

I think the best we can do is to support societies that have minimal violence, that work practically, and that generally conform to people's sense of fairness. I also think history is rife with land grabs that seem barbaric. But, in practical terms, its usually too difficult to litigate these ancient acts of barbarity. So, we just have to accept the violence of the past, and strive to ensure that it doesn't happen again.

I suppose I mainly took issue with your affirmative claim that "conquering" land is legitimate. Its not legitimate. Its just too difficult to undo. We have societal infrastructure built on land that was stolen from dead people whose names have been lost to history, and who may have stolen the land themselves. There's no way to rectify that. We have no choice but to live with it. But, that doesn't make it "legitimate".