The United States was born of our ancestors' nationalistic resentment of a foreign power whose troops we saw as occupiers, not protectors. The British never fathomed our basic grievance -- this was our land, not theirs! -- so the more they cracked down, the more they empowered the American insurgency.
Given that history, you'd think we might be more sensitive to nationalism abroad. Yet the most systematic foreign-policy mistake we Americans have made in the post-World War II period has been to underestimate its potency, from Vietnam to Latin America.