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The murder of four Americans in Libya and mob assaults on U.S. embassies across the Muslim world this month have reminded many of 1979, when radical Islamists seized the American mission in Tehran. There, too, extremists running wild after the fall of a pro-American tyrant had found a cheap way of empowering themselves.

But the obsession with radical Islam misses a more meaningful analogy for the current state of siege in the Middle East and Afghanistan: the helicopters hovering above the roof of the American Embassy in Saigon in 1975 as North Vietnamese tanks rolled into the city.