Ever since the United States emerged as an imperial power at the end of World War II, it has needed at least one enemy, preferably more. For many years, the Soviet Union played this role to perfection. But the fall of the Berlin Wall in 1989 created something of a vacancy. And after a series of auditions from possible candidates (Libya, Iraq, and so forth), Iran has come to fill the bill.

In truth, Iran poses no conceivable threat. In 2008, the last year for which we have reliable figures, the country spent less than 1 percent of the…