On Tuesday, Iranian President Mahmoud Ahmadinejad did not just reject President Obama's latest feckless floating nuclear deadline. He spat on it, declaring that Iran "will continue resisting" until the U.S. has gotten rid of its 8,000 nuclear warheads.

So ends 2009, the year of "engagement," of the extended hand, of the gratuitous apology — and of spinning centrifuges, two-stage rockets and a secret enrichment facility that brought Iran materially closer to becoming a nuclear power.

We lost a year. But it was not just any year. It was a year of spectacularly squandered opportunity. In Iran, it was a year of revolution, beginning with a contested election and culminating this week in huge demonstrations mourning the death of the dissident Grand Ayatollah Hossein Ali Montazeri — and demanding no longer a recount of the stolen election but the overthrow of the clerical dictatorship.

Obama responded by distancing himself from this new birth of freedom. First, scandalous silence. Then, a few grudging words. Then relentless engagement with the murderous regime. With offer after offer, gesture after gesture — to not Iran, but the "Islamic Republic of Iran," as Obama ever so respectfully called these clerical fascists — the U.S. conferred legitimacy on a regime desperate to regain it.

Why is this so important? Because revolutions succeed at that singular moment, that imperceptible historical inflection, when the people, and particularly those in power, realize that the regime has lost the mandate of heaven. With this weakening dictatorship desperate for affirmation, why is the U.S. repeatedly offering just such affirmation?

Apart from ostracizing and delegitimizing these gangsters, we should be encouraging and reinforcing the demonstrators. This is no trivial matter. When pursued, beaten, arrested and imprisoned, dissidents can easily succumb to feelings of despair and isolation. Natan Sharansky testifies to the electric effect Ronald Reagan's Evil Empire speech had on lifting spirits in the Gulag. The news was spread cell to cell in code tapped on the walls. They knew they weren't alone, that America was committed to their cause.

Yet so aloof has Obama been that on Hate America Day (Nov. 4, the anniversary of the seizure of the U.S. Embassy in Tehran), pro-American counter-demonstrators chanted "Obama, Obama, you are either with us or with them," i.e., their oppressors.

Such cool indifference is more than a betrayal of our values. It's a strategic blunder of the first order.

Forget about human rights. Assume you care only about the nuclear issue. How to defuse it? Negotiations are going nowhere, and whatever U.N. sanctions we might get will be weak, partial, grudging and late. The only real hope is regime change. The revered and widely supported Montazeri had actually issued a fatwa against nuclear weapons.

And even if a successor government were to act otherwise, the nuclear threat would be highly attenuated because it's not the weapon but the regime that creates the danger. (Think India or Britain, for example.) Any proliferation is troubling, but a nonaggressive pro-Western Tehran would completely change the strategic equation and make the threat minimal and manageable.

What should we do? Pressure from without — cutting off gasoline supplies, for example — to complement and reinforce pressure from within. The pressure should be aimed not at changing the current regime's nuclear policy — that will never happen — but at helping change the regime itself.

Give the kind of covert support to assist dissident communication and circumvent censorship that, for example, we gave Solidarity in Poland during the 1980s. (In those days that meant broadcasting equipment and copying machines.) But of equal importance is robust rhetorical and diplomatic support from the very highest level: full-throated denunciation of the regime's savagery and persecution. In detail — highlighting cases, the way Western leaders adopted the causes of Sharansky and Andrei Sakharov during the rise of the dissident movement that helped bring down the Soviet empire.

Will this revolution succeed? The odds are long but the reward immense. Its ripple effects would extend from Afghanistan to Iraq (in both conflicts, Iran actively supports insurgents who have long been killing Americans and their allies) to Lebanon and Gaza where Iran's proxies, Hezbollah and Hamas, are arming for war.

One way or the other, Iran will dominate 2010. Either there will be an Israeli attack or Iran will arrive at — or cross — the nuclear threshold. Unless revolution intervenes. Which is why to fail to do everything in our power to support this popular revolt is unforgivable.

Charles Krauthammer writes for the Washington Post Writers Group. Reach him at letters@charleskrauthammer.com.