Iran is a signatory to the non-proliferation treaty, under which it is entitled to conduct nuclear research for peaceful purposes. Despite the offer of trade agreements and the prospect of military action, it defied a demand by the I.A.E.A. and the Security Council, earlier this year, that it stop enriching uranium—a process that can produce material for nuclear power plants as well as for weapons—and it has been unable, or unwilling, to account for traces of plutonium and highly enriched uranium that have been detected during I.A.E.A. inspections. The I.A.E.A. has complained about a lack of “transparency,” although, like the C.I.A., it has not found unambiguous evidence of a secret weapons program.

Last week, Iran’s President, Mahmoud Ahmadinejad, announced that Iran had made further progress in its enrichment research program, and said, “We know that some countries may not be pleased.” He insisted that Iran was abiding by international agreements, but said, “Time is now completely on the side of the Iranian people.” A diplomat in Vienna, where the I.A.E.A. has its headquarters, told me that the agency was skeptical of the claim, for technical reasons. But Ahmadinejad’s defiant tone did nothing to diminish suspicions about Iran’s nuclear ambitions.

“There is no evidence of a large-scale covert enrichment program inside Iran,” one involved European diplomat said. “But the Iranians would not have launched themselves into a very dangerous confrontation with the West on the basis of a weapons program that they no longer pursue. Their enrichment program makes sense only in terms of wanting nuclear weapons. It would be inconceivable if they weren’t cheating to some degree. You don’t need a covert program to be concerned about Iran’s nuclear ambitions. We have enough information to be concerned without one. It’s not a slam dunk, but it’s close to it.”

There are, however, other possible reasons for Iran’s obstinacy. The nuclear program—peaceful or not—is a source of great national pride, and President Ahmadinejad’s support for it has helped to propel him to enormous popularity. (Saddam Hussein created confusion for years, inside and outside his country, about whether Iraq had weapons of mass destruction, in part to project an image of strength.) According to the former senior intelligence official, the C.I.A.’s assessment suggested that Iran might even see some benefits in a limited military strike—especially one that did not succeed in fully destroying its nuclear program—in that an attack might enhance its position in the Islamic world. “They learned that in the Iraqi experience, and relearned it in southern Lebanon,” the former senior official said. In both cases, a more powerful military force had trouble achieving its military or political goals; in Lebanon, Israel’s war against Hezbollah did not destroy the group’s entire arsenal of rockets, and increased the popularity of its leader, Hassan Nasrallah.

The former senior intelligence official added that the C.I.A. assessment raised the possibility that an American attack on Iran could end up serving as a rallying point to unite Sunni and Shiite populations. “An American attack will paper over any differences in the Arab world, and we’ll have Syrians, Iranians, Hamas, and Hezbollah fighting against us—and the Saudis and the Egyptians questioning their ties to the West. It’s an analyst’s worst nightmare—for the first time since the caliphate there will be common cause in the Middle East.” (An Islamic caliphate ruled the Middle East for over six hundred years, until the thirteenth century.)

According to the Pentagon consultant, “The C.I.A.’s view is that, without more intelligence, a large-scale bombing attack would not stop Iran’s nuclear program. And a low-end campaign of subversion and sabotage would play into Iran’s hands—bolstering support for the religious leadership and deepening anti-American Muslim rage.”

The Pentagon consultant said that he and many of his colleagues in the military believe that Iran is intent on developing nuclear-weapons capability. But he added that the Bush Administration’s options for dealing with that threat are diminished, because of a lack of good intelligence and also because “we’ve cried wolf” before.