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. On the contrary, by the millions they have gone to considerable lengths over the last four months to make one thing clear: When affixing responsibility for the misery, shame and danger being visited on their once-great nation, they focus overwhelmingly on the ruling regime itself — on its economic incompetence, its tyrannical nature, its international belligerence.

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.

Almost any outside action that further squeezes Iran's tyrants and calls into question their legitimacy in the eyes of the world will be welcomed, even at the risk of imposing additional hardships on the Iranian people. That was certainly the message I heard at a recent gathering of Iranian activists in Europe, including figures closely linked to the green movement's leadership. Sanctions must be imposed, and in strong doses, the group urged. A weak dose, or gradual approach, only allows the regime to adjust, they said. To be effective, sanctions must act like a shock, not a vaccine.

Similarly, prominent Iran expert Karim Sadjadpour told a Washington conference last month: "Whereas in the past (the leaders of Iran's opposition) were ... unequivocally opposed to any type of punitive measures by the United States ... that's not the case anymore."

While it remains too risky for the opposition's leadership to call publicly for sanctions, Sadjadpour claimed that privately they are eager to discuss what measures would be most effective and to synchronize their activities with U.S. actions against the regime.

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.

Accurately assessing how these different scenarios will play out is crucial for U.S. interests. The stakes could not be higher, and the answers are far from certain. But 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.