The huge, nonviolent crowds, and their chants (”Dictator, this is your last message: The people of Iran are rising!”), proved that there is still plenty of life in the popular movement that Mr. Khamenei and his Revolutionary Guards provoked by engineering Mr. Ahmadinejad’s fraudulent reelection in June. Given the horrific extent of the repression against that movement, its continued energy is nothing short of inspiring.

But Mr. Montazeri had also linked the democratization of Iran to its peaceful coexistence with the West. Before his death, he apologized for the 1979 Iranian seizure of the U.S. Embassy in Tehran and — undoubtedly most irritating to Mr. Khamenei — opposed the regime’s nuclear ambitions.

The most momentous international event of 2009 was the uprising in Iran, and though the regime’s collapse is not imminent, it is hardly unthinkable. President Obama is prudent to pursue a diplomatic solution to Iran’s nuclear ambitions. But in doing so, he must not diminish the prospect that Iran’s people might ultimately deliver both themselves and the world from the menace.