Without 9/11, Iran, another chief U.S. rival in the region, almost certainly would wield less influence than it does today. America’s wars in Iraq and Afghanistan removed two of Iran’s major enemies — Hussein, who had waged war against Iran, and the Taliban, a Sunni radical regime that despised Iran’s Shiite mullahs. And the destruction of Hussein’s Sunni-minority regime has allowed Iran to greatly increase its influence among Iraq’s Shiite majority.

And what of the Arab Spring? How did 9/11 affect this movement? It seems clear that the timing of the revolts in the Arab world has had more to do with the long-term decay of sclerotic dictatorships than with anything the jihadist movement has offered — in part because its atrocities against Muslims as well as against “crusaders” and “Jews” have disgusted many Muslims. Paradoxically, if Muslim radicalism had been less tainted by the mass murder of 9/11, its long-term goal of replacing Arab regimes with Muslim theocracies might have had more appeal among the region’s revolutionaries.

On the home front, American politics would have unfolded in vastly different ways. Assume that, with no 9/11 to use as a pretext to invade Iraq, the election of 2004 would have been a peacetime contest, centered largely on economic issues. It may seem minor compared with today’s economic difficulties, but recall how a “jobless recovery” followed the crash of the tech bubble, and the subsequent bubble in real estate and stocks took time to inflate. George W. Bush, like his father before him, might not have won reelection if the Democratic nominee had run a reprise of the 1992 Clinton campaign, with the message “it’s the economy, stupid.”