Nations -- because they operate in a competitive and sometimes dangerous world -- seek to preserve their identities and cultural values. In many cases, the best way for them to do that is to have their own state, because ethnic or national groups that lack their own state are usually more vulnerable to conquest, absorption, and assimilation.The question is, how many subcultures and "subnarratives" (or alternative narratives) have been obliterated in this competition of nation states? It's a Darwinian battle of natural selection, in which the groups with the right combination of strength of cultural narrative, strength of military power, and luck emerge as stable political entities.
Similarly, modern states also have a powerful incentive to promote national unity -- in other words, to foster nationalism -- because having a loyal and united population that is willing to sacrifice (and in extreme cases, to fight and die) for the state increases its power and thus its ability to deal with external threats. In the competitive world of international politics, in short, nations have incentives to obtain their own state and states have incentives to foster a common national identity in their populations. Taken together, these twin dynamics create a long-term trend in the direction of more and more independent nation-states.
Optimistically, what may emerge in the long run is a peaceful humanism, but even if it did, would we have any idea what had been lost in the process? Realistically, what will probably emerge will be far from peaceful.
In any case, I'll just make the assertion that I think nationalism has no place in a Christian set of morals, nor in any humanist set of morals. The idea that people exist to preserve a particular national identity is an assault in the inherent dignity of humans as individuals. It is made all the worse by the violent manner in which nationalism tends to manifest itself. American nationalism is no exception.