In an ideal world we could celebrate each other's differences. But trying to get rid of conflict by getting rid of national borders is naive. Why are the borders where they are? Generally because those are ethno-cultural boundaries. Nations that encompass multiple ethno-cultural groups tend to be somewhat unstable: for instance, Yugoslavia broke up violently, and Iraq has conflict between the Kurds and the rest.
This is not a support of nationalism (although I encourage patriotism, which is different), but "countries are [nothing] more than a shared historical hallucination" is just incorrect.
