Here's another argument from Jorge Gracia's recent book Surviving Race, Ethnicity, and Nationality: A Challenge for the Twenty-first Century. (See here for the first.) This argument (which I should make clear Gracia does not endorse) is against nationality on the grounds that in a nation you have conflicts between national obligations and universal duties to humanity. Special obligations to protect fellow members of your own nation will potentially (and in fact might often) lead to actions that harm or kill members of other nations. According to this argument, then, it's wrong to allow a setting for such conflicts, which means we need to remove the idea of nationality altogether.
Conflicts between general and special obligations occur all the time, and it's not a reason to remove the special obligations. If I have an obligation to provide for my family, that means I have fewer resources to use for any general obligations I have to help out the poor of my community or of the world. If I have a special obligation to defend my son from a violent attacker, that means I might have to harm or kill the attacker. In the first case, this conflict of obligations doesn't mean I should stop thinking in terms of a family in order to prevent the conflict. In the second, the conflict is only illusory to begin with. I have no general obligation not to harm someone who is trying to harm my son, if the harm I do is necessary to prevent harm to my son. I have a prima facie obligation not to harm people, but that obligation is trumped by other considerations when my son's wellbeing is severely threatened by an evil-doer. By parity of reasoning, the same is true of nations and those in nations defending their fellow members of their nation against outside attackers in the conflict is just, and unjust wars are unjust and thus not justified to begin with, so those don't raise a conflict of obligations unless you're in the military, and then your carrying out orders is usally seen as excused because of the nature of military decision-making. I just don't see how this is supposed to count as a consideration against the existence of nations or the idea of nationality at all.