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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.

We have been similarly oblivious to the strength of nationalism in Afghanistan and Pakistan, particularly among the 40 million Pashtuns who live on both sides of the border there. That’s one...