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.
Obviously, nations and states don't always reach the goal of a unified "nation-state." Some nations never succeed in gaining independence, and some states never succeed in creating a unified national identity for themselves. And not every cultural or ethnic group thinks of itself as a nation or aspires to independence (though one can never be sure when some group might begin to acquire "national consciousness" and head in that direction). Nonetheless, the past hundred years has seen a steady rise in the number of states and the emergence of strong national movements in many of them, and I see no reason to expect this trend to be reversed.
Once established, a nation-state is a self-reinforcing phenomenon. Nation-states are hard to conquer and subdue, because the local population will usually resist outside invasion and continue to struggle against a foreign occupier. Successful national movements tend to spawn imitators, leading to further demands for statehood. Despite its occasional shortcomings (and the obvious examples of "failed states" like Somalia, Yemen, or Afghanistan), the national state is likely to remain the most important political entity in world politics for the foreseeable future.
Because American national identity tends to emphasize the civic dimension (based on supposedly universal principles such as individual liberty) and tends to downplay the historic and cultural elements (though they clearly exist) U.S. leaders routinely underestimate the power of local affinities and the strength of cultural, tribal, or territorial loyalties. During the Cold War, we persistently exaggerated the strength of transnational ideologies like Communism, and underestimated the degree to which national identities and interests would eventually generate intense conflicts within the Marxist world. Osama bin Laden made the same mistake when he thought that terrorist attacks and video-taped fulminations would ignite a mass movement to re-establish a transnational Islamic caliphate. And anyone who thinks that a rising China is going to tamely submit to U.S. or Western notions of the proper world order fails to appreciate the degree to which nationalism is also a central part of the Chinese worldview, and far more important than any lingering "communist" ideals.
Unless we fully appreciate the power of nationalism, in short, we are going to get a lot of things wrong about the contemporary political life. It is the most powerful political force in the world, and we ignore it at our peril.