Military strikes against nuclear installations in Iran would be difficult; they could provoke retaliation and would certainly result in international condemnation. But Israeli officials argue that the backlash would be less painful than allowing Iran to obtain nuclear weapons.
Inspectors have examined research centers and workshops across the country, interviewed hundreds of scientists and pored over thousands of pages of documents dating to the mid-1980s, when Iran began secretly buying nuclear technology.
At every step, Iran concealed crucial aspects of its program. Iranian authorities twice denied inspectors access to suspect locations while incriminating material and equipment was hauled away. Each time, the inspectors said they still found evidence of nuclear experiments.
"The world at large knew nothing about Iran's nuclear plans two years ago and everything since then has been pried out of them," a U.S. diplomat said.
Under pressure from Washington, the IAEA board last month told the agency's staff to conduct another round of inspections and prepare a comprehensive summary of findings.
The board also ordered Iran to stop its enrichment program. Tehran voluntarily curtailed enrichment in a deal made a year ago with Britain, France and Germany, but it resumed the work this year.
The IAEA summary report will be circulated two weeks before representatives of the 35 member nations on the board meet Nov. 25 in Vienna. The expected conclusion that there is no proof of a weapons program and no new evidence of concealment is unlikely to stop the United States from demanding a vote to refer the issue to the Security Council.
Iran escaped previous U.S. pushes for tough action. That's unlikely this time, diplomats say, unless Iran again halts enrichment — and even that is no guarantee it can avoid referral.
Diplomats familiar with the U.S. strategy said U.N. sanctions would be the first step in an effort to force Tehran to abandon enrichment efforts. Harsher steps eventually could include military action.
But other diplomats said Russia, China and other governments were reluctant to endorse sanctions, worrying that they might be the first step leading to an attack on Iran.
Tehran's best chance of avoiding being hauled before the Security Council appears to be accepting a new European offer of a package of incentives that would include guaranteeing Iran access to civilian nuclear technology and fuel for a nearly complete reactor in exchange for the country mothballing its enrichment efforts.
Tehran has not responded formally to the proposal, which is to be presented to its representatives today in Vienna. Western diplomats expect some concession before the Nov. 25 meeting.
Foreign Minister Kamal Kharrazi said in an interview last month that Tehran was willing to consider "any kind of verification mechanism to make sure there is no secret program." He said the goal of any agreement with the Europeans would be to prevent the issue from going to the Security Council.
Iran's conservatives, who have solidified control of the government since the first agreement with the Europeans, appear divided over whether to strike another bargain.
Defiant hard-liners want to withdraw from the Nuclear Nonproliferation Treaty and go ahead with enrichment. Proposed legislation would require Iran to pull out of the accord and halt IAEA inspections.
"Countries are seriously concerned about Iran withdrawing from the treaty," a Western diplomat said. "That's the second-worst scenario — getting the bomb being the worst."
IAEA chief ElBaradei warned that Iran's withdrawal could prompt other countries to follow, severely damaging the primary means of controlling the spread of nuclear weapons.
More moderate voices in Iran argue that the country should remain in the treaty and try to avoid sanctions by accepting the European deal.