SPIEGEL ONLINE: Iran has successfully produced enriched uranium, but it has not yet built a nuclear bomb. What do they still need to do to build a bomb?

Ladurner: There are a number of technical steps the Iranians have not yet mastered. At present, they claim to be able to produce uranium that has been enriched by 3.5 percent; to build a nuclear bomb, they would need 90 percent. It's not as if they wouldn't be able to do so; their problem is that they would have to produce large quantities of such highly enriched uranium, which they're not yet able to do. Then, when they've succeeded in installing a system of centrifuges, they still need to construct the warheads, and make them small enough to be used in combat.

SPIEGEL ONLINE: How close to building a bomb is Iran?

Ladurner: If they really want to build one, they'll need at least another five years.

SPIEGEL ONLINE: Where could Iran get the know-how it needs?

Ladurner: Iran's scientists now have enough material and technical expertise to continue on their own. Whether they know enough to build sufficiently small warheads is still unclear. It's also hard to tell whether the international smuggler's ring associated with the father of the Pakistani nuclear bomb, Abdul Quadir Khan, is still active; Khan is under house arrest in Pakistan. I don't think all the gaps have been filled.

SPIEGEL ONLINE: The two sides in the dispute over Iran's nuclear program are increasingly entrenched, with neither trusting the other. Iran has tried to fool the international community too many times. Can one really predict what the regime will do next?

Ladurner: At the moment, the regime's main concern is its own survival. The leadership knows it's not very popular among the Iranians; it's aware of the country's economic problems and knows that it is isolated internationally. What's more, there's an ongoing struggle between conservatives and reformers in Iran. They have to divide the spoils between each other. Most of the imponderables lie within the political system itself.

SPIEGEL ONLINE: How can one gauge the population's disapproval of its government?

Ladurner: Iranians are openly criticizing their government. Before Ahmadinejad was elected president, there was an entire political movement threatening to boycott the elections in order to delegitimize the political system. Last but not least, 200,000 young Iranians leave the country every year because they see no future for themselves there.

SPIEGEL ONLINE: But when it comes to the issue of nuclear power, Iranians support their government.

Ladurner: This is a card that the regime has played very cleverly. Iranians are asking themselves why the West denies Iran the right to use nuclear power for civilian purposes, a right enshrined in the Non-Proliferation Treaty. They feel that their national pride has been wounded. So they don't so much sympathize with the regime as feel they are in the same boat with it.

SPIEGEL ONLINE: Is the government trying to smooth over internal disputes by creating a permanent crisis around the nuclear program?

Ladurner: The regime is hoping to win the support of the general population by creating an outside enemy. That's the goal behind Ahmadinejad's provocative statements about Israel. Conversely, this means that the regime is weak. It's a paradoxical situation: the regime feels strong and weak at the same time.

SPIEGEL ONLINE: Various developments are conceivable, some of them more and some of them less desirable. What would be the consequences of Iran becoming a nuclear power?

Ladurner: Several countries in the Middle East, the Near East and northern Africa would set out to improve their own defense capabilities. Iran would become the leading power in the region. It would become militarily unassailable -- a goal it was already pursuing prior to the Iranian revolution. At the same time, Iran would be isolated internationally for some time, like North Korea. The results inside the country are hard to predict: Iran is not a stable state, and further instability combined with nuclear capabilities would be a disaster.

SPIEGEL ONLINE: Preventive military strikes are still an option. The United States is already talking about a possible "coalition of the willing."

Ladurner: It wouldn't be enough to just destroy Iran's existing nuclear facilities. That would certainly set the Iranians back, but it wouldn't stop them, given the technical expertise they've already acquired.

SPIEGELONLINE: What would the political consequences be?

Ladurner: Notwithstanding the fact that many Iranians disapprove of it, the regime would be strengthened. Iran would be sure to defend itself, using its influence in Iraq and Lebanon and possibly blocking the Strait of Hormus in the Persian Gulf, which is vital for international oil shipments. There are many things Iran could do.

SPIEGEL ONLINE: Which leaves sanctions. What could an intelligent sanctions policy look like?

Ladurner: Sanctions have to affect the right people. That's easily said, of course. And you need enough allies for the sanctions to be imposed effectively. Even if travel prohibitions, export limitations and import controls can't isolate Iran entirely, they have a symbolic character. Until now, the attitude of the Iranian regime has always been: "There's nothing they can do to us because they can't even agree among themselves." Sanctions might be combined with a diplomatic offer to Iran -- a combination of the carrot and the stick.

SPIEGEL ONLINE: None of that seems very realistic at present. Russia and China have said they wouldn't back sanctions. And in March the US boycotted a proposal for a compromise that would have allowed Iran to experiment with centrifuges at the nuclear site in Natans while making sure that uranium is enriched only on Russian soil.

Ladurner: But what are the alternatives? The Russian option is really the only viable one. It guarantees Iran the right to enrich uranium and ensures that this is not done for military purposes. Sooner or later, the US will have to face reality.

SPIEGEL ONLINE: The UN ultimatum expires Friday. Is the Security Council the right place for resolving the conflict over Iran's nuclear program?

Ladurner: When the conflict was taken from the International Atomic Energy Agency to the UN Security Council, there was a hope that this would open up new options. Now it looks like we've ended up in a cul de sac. Because what comes after the Security Council? Either nothing at all -- or a "coalition of the willing."

Interview conducted by Philipp Wittrock