If current negotiations falter, international efforts to curtail Iran’s nuclear program may escalate to the imposition of “crippling sanctions” or even the use of military force. A crucial question that policymakers must ask is whether such punitive measures would help or hinder the popular uprising against the Iranian regime that emerged after the country’s fraudulent June 12 presidential elections.

The so-called green movement — the color has been adopted by the opposition — poses the most serious challenge to the survivability of the Islamic Republic in its 30-year history. Few analysts doubt that if it succeeded in toppling Iran’s hard-line regime, the crisis over the Iranian nuclear program would become far more susceptible to diplomatic resolution.

Before June 12, conventional wisdom suggested that both harsh sanctions and military action probably would strengthen the Islamic Republic by triggering a “rally around the regime” effect.

Iran’s rulers, so the claim went, would exploit outside pressure to stoke Persian nationalism, deflecting popular anger away from the regime’s own cruelty onto the perceived foreign threat — in effect, short-circuiting the country’s incipient democratic revolution.

But the conventional wisdom has taken something of a beating post-June 12. Before the elections, Iranian President Mahmoud Ahmadinejad sought to blame all of Iran’s travails — a deteriorating economy, international isolation, the mounting threat of war — on the United States and Israel. But the Iranian people were buying none of it.

There’s good reason to doubt Iranians would react differently now were the United States and its partners to impose painful sanctions. If anything, the bloody crackdown the Iranian people have endured since the election has only fueled their hatred of the current ruling clique and their determination to be rid of it as soon as possible.

What about military action? This is a much harder call. Iran experts are split. The majority still maintain that Iranians would quickly unite to confront any foreign attacker. While opposition representatives I heard in Europe think that’s unlikely, they are deeply worried that if the regime is not crippled in any military attack, it will move ruthlessly to crush their movement for good.

But a few Iranians — especially in private — see other possibilities. They suggest that a bombing campaign that spared civilians while destroying Iran’s nuclear installations as well as targets associated with the regime’s most repressive elements — the Revolutionary Guard and Basij militia — might well accelerate the theocracy’s final unraveling at the hands of an already-boiling population.

Accurately assessing how these different scenarios will play out is crucial for U.S. interests. It does seem likely that the international community’s room for maneuvering may be far more extensive than many people believed before the summer’s uprising. Just how extensive should be the subject of urgent review by the United States and its allies as they seek to ensure that the Islamic Republic’s unprecedented domestic vulnerability is fully exploited to stop its dangerous march toward nuclear weapons.

Hannah served as national-security adviser to former Vice President Dick Cheney from 2005 to 2009. A version of this commentary appeared in Thursday’s Los Angeles Times.