16 07 11 - 15:15From an informed source:
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. - Foreign Policy
Nationalism is a powerful force because unlike politics, it relates to who you are and what real-world values you have, not some airy preference for whatever political-economic system you have to keep the proles in line.