I think this is a legitimate question for why so many conflicts and wars among/between countries in human history.
As mentioned before, I don't think a world citizen approach (globalism/globalisation) is really feasible or workable, practically. For practical purpose, there should still be borders, as also mentioned. However, in addition to World-State Borders (cosmopolitanism), there would be also Continental Borders (e.g. Africa Border), or Partial-Continental Borders (e.g. North America Border) - which is designed mainly for border control purpose. The conventional National Borders then would be substituted by these two layers of new borders. LOL
LOL is right. This is swimming against the tide. The tide towards control returning more and more to the local level. Less, not more globalism.