Mr. Abbasi, according to people familiar with the I.A.E.A.’s investigation, worked on calculations on increasing the yield of nuclear explosions, among other problems in manufacturing a weapon. He was a key scientist in the Iranian covert nuclear weapons program headed by Mohsen Fakhrizadeh, an academic and strong supporter of Iran’s nuclear weapons program. For the past decade, Mr. Fakhrizadeh has run programs — with names like “Project 110” and “Project 5,” they seem right out of a James Bond movie — that the West believes are a shell game hiding weapons work. Suspicions have been heightened by Iran’s refusal to allow him or his colleagues to be interviewed by the United Nations’ nuclear inspection teams. And since last year’s attacks — and another this past summer — Mr. Fakhrizadeh has gone completely underground.

No one expects the United Nations’ revelations of the evidence to prompt more action against Iran. Most governments have had access to this evidence for a while. The Iranians will say it is all fabrication, and because the agency will not reveal its sources, that charge could stick. The Chinese and the Russians have already protested to the I.A.E.A. head, Yukiya Amano, that revealing the evidence will harden Iran’s position. They oppose any new sanctions.

While the Obama administration may act unilaterally to shut down transactions with Iran’s central bank, officials concede that the only economic step that could give the mullahs pause would be a ban on Iranian oil exports. With oil already hovering around $93 a barrel, no one in the administration is willing to risk a step that could send prices soaring and, in the worst case, cause a confrontation at sea over a blockade.

For all the talk about how “all options are on the table,” Washington says a military strike isn’t worth the risk of war; the Israelis say there may be no other choice. But they have said “this is the last chance” every year since 2005.

All of which raises the question: how much more delay can be bought with a covert campaign of assassination, cyberattacks and sabotage?

Some more, but probably not much. It has taken the Iranians 20 years so far to get their nuclear act together — far longer than it took the United States and the Soviets in the ’40s, the Chinese and the Israelis in the ’60s, the Indians in the ’70s, and the Pakistanis and the North Koreans in more recent times. The problem is partly that they were scammed by Abdul Qadeer Khan, the Pakistani who sold them his country’s discards.

The assassination and the sabotage have taken a psychological toll, making scientists wonder if every trip to work may be their last, every line of code the beginning of a new round of destruction. Stuxnet was devilishly ingenious: it infected millions of computers, but did damage only when the code was transferred to special controllers that run centrifuges, which spin at supersonic speed when enriching uranium. When operators looked at their screens, everything looked normal. But downstairs in the plant, the centrifuges suddenly spun out of control and exploded, like small bombs. It took months for the Iranians to figure out what had happened.

But now the element of surprise is gone. The Iranians are digging their plants deeper underground, and enriching uranium at purities that will make it easier to race for a bomb. When Barack Obama was sworn into office, they had enough fuel on hand to produce a single weapon; today, by the I.A.E.A.’s own inventory, they have enough for at least four. And as the Quds Force has shown, sabotage and assassination is a two-way game, which may ratchet up one confrontation just as Americans have been exhausted by two others.