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As President Obama decides how to proceed in the Afghan war, he needs to add one more variable that is rarely mentioned: Iranian determination to acquire nuclear weapons. An ongoing Afghanistan campaign means that resort to force against Iran would be tantamount to starting a second war. The politics being what they are, that will knock the military option against Iran off the table, with negative implications for an empowered diplomacy toward Iran.

Consider the timelines of the Afghan and Iranian policy portfolios, as President Obama must. Whether or not Iran parts with some of its fissile material in coming months in accord with the recent Geneva deal, it will still have enough nuclear “stuff” for one at least bomb within 18 months. (It may have more than that if,...