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The new plan that President Obama laid out for a missile shield against Iran on Thursday turns Ronald Reagan’s vision of a Star Wars system on its head: Rather than focusing first on protecting the continental United States, it shifts the immediate effort to defending Europe and the Middle East.

It is a long way from the impermeable shield that President Reagan described in glowing terms in 1983, an announcement that turned into a diplomatic triumph even while it was a technological flop. Ever since, missile defense has always been more about international politics than about new military technology.

In the last years of the cold war, it helped nudge the Soviets toward agreements that sharply reduced nuclear arsenals, a process that Mr. Obama hopes to revive at the end...