No one ever said it was going to be easy. The question is: Do we have the time? Iran is once again confounding hopes, and well-laid plans, for a significant start in dealing with its nuclear aspirations. Iran failed to meet a Friday deadline to sign on to shipping strategically important uranium out of the country. The Iranians said they would give their answer next week. The United States, France and Russia, the three countries negotiating the deal, accepted the delay.

This is a disappointment but not a deal-breaker. Talks are still on, engagement is still on. But perpetual delays could derail this process. At a certain point, the Obama administration would judge that that Iran has no intention to reassure the world about its nuclear program, which has already produced enough enriched uranium to serve as the base material for making a first nuclear bomb.

If that pattern is setting in again, then Obama’s bright new engagement policy has no chance of succeeding.

Obama’s policy of engagement has three purposes. The first is actually to work with Iran, instead of threatening it since confrontation did not help the Bush administration advance its goals. The second is that engagement tests Iran’s intentions. Are the Iranians able to say yes to a good offer, or will they simply not compromise on any aspect of their nuclear program? This clears they way to the third purpose: unite the the international community, specifically Iranian allies Russia and China, behind tough measures, ranging from sanctions to military action, if diplomacy doesn’t work.

• Michael Adler: Iran’s Big Test “They have to make up their minds,” a diplomat tells me. “They say all the time that the program is about peaceful intentions and peaceful use, and the whole subject of the proposal is to help them with that peaceful program.” Iran is being offered a chance to ship most of its stockpile of low-enriched uranium, fissile material that can become bomb material, to Russia. Russia would refine it further for use as fuel, although not enriched enough for a bomb. This enriched uranium would be shipped to France to be made into fuel rods for a research reactor in Tehran that makes radio-isotopes for medical diagnostic procedures.

Iran gets something. It would get the fuel and it would still be making enriched uranium, which it says is only for peaceful use in civilian research and power reactors. But it would have shipped out 1,200 kilograms of the estimated 1,500 kilograms it has, and so would not have enough left to make a bomb. A total of 1,500 kilograms of low-enriched uranium is needed for further refinement to a weapons-grade amount. Iran would need at least a year, at current rates of production, to get its stockpile back up to the 1,500-kilogram level. Iranian negotiators agreed in principle to this in Geneva on October 1, with details to be worked out at a meeting that began Monday in Vienna.

Then Iran said it would not work with France, which in the past has refused to send it enriched uranium. The meeting ended only when U.N. nuclear watchdog chief Mohamed ElBaradei presented the proposal in the name of his International Atomic Energy Agency, and therefore the international community. This meets Iranian demands. (Iran has repeatedly said it insists on working with the IAEA on nuclear matters.)

Iran is now hinting it wants to go back to its original proposal, which was for the IAEA to help it buy the enriched uranium it needs for its research reactor. This would torpedo the deal the United States and other world powers are trying to strike, which is centered around reducing Iran’s uranium stockpile.

The test of Iran’s intentions is now more critical than before. Adding to the concern over the enrichment pact is an inspection Sunday and Monday by the IAEA of a secret Iranian enrichment site that was only revealed recently. The IAEA is asking for access not only to the site but to building and design records at the plant, which is not yet operating, and to interviews with scientists involved. Iran has for years delayed, and sometimes obstructed, in supplying the IAEA with such information at other sites.

If that pattern is setting in again, then Obama’s bright new engagement policy has no chance of succeeding. And if engagement does not succeed, and if the Iranians are determined—as Washington suspects and Tehran denies—to have nuclear weapons, then Obama is faced with a stark choice. It is a choice that diplomacy is designed to keep the president from having to face.

Michael Adler, a longtime reporter for Agence France-Presse, is a scholar at the Woodrow Wilson Center and is writing a book on Iran’s nuclear diplomacy, which he has covered for most of this decade.