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President Barack Obama and other world leaders agreed two months ago that at the G-20 summit in Pittsburgh this month they would "re-evaluate Iran's posture towards negotiating the cessation of a nuclear weapons policy." That reassessment cannot come too soon.

Last year, a high-level Bipartisan Policy Center task force in which we participated concluded that a nuclear weapons-capable Iran would be "strategically untenable." Alarmed by how little diplomatic progress has been made, we have just updated that report. Not only has Iran continued its nuclear program unabated, but its regime has emerged from post-election turmoil more radical than ever.

From his first days in office, Mr. Obama has extended an open hand to Iran. His efforts...