Even the administration seems tentative about when Iran will exceed American tolerance. In the Pentagon and the intelligence agencies, several senior officials complain — though never on the record — that President Obama and his staff have not clearly defined when Iran will gain a “nuclear weapons capability.” Many argue that similar indecision preceded the day in 2006 when Mr. Bush woke up to discover that North Korea had conducted a nuclear test.

So what is the argument for containment? Basically, it assumes that if China and Russia changed over decades, so might Iran. And nuclear weapons can handcuff a nation as easily as they can empower it. Last week, at the University of Oklahoma, Mr. Brzezinski argued that either an Iranian bomb or an attack on Iran would be “a calamity, a disaster.” He said containment could work because Iran “may be dangerous, assertive and duplicitous, but there is nothing in their history to suggest they are suicidal.”

Nevertheless, in their Foreign Affairs essay, James Lindsay and Ray Takeyh concede that the Iran case differs substantially from the cold war ones, and that a successful strategy today would have to recognize that fact. They urge Mr. Obama to prescribe three explicit no-go zones for the Iranians: “no initiation of conventional warfare” against another nation; “no transfer of nuclear weapons, materials, or technologies”; no increase in support for terrorists. The penalty, they argued, would have to include “military retaliation by any and all means necessary,” including the use of nuclear weapons.

It is a logical list. But there is a counterargument: Why would Iran believe the threat if the United States, having said it would never allow Iran to get a nuclear capability, then allowed it?

In fact, the administration is deep into containment now — though it insists its increases in defensive power in the Gulf are meant to deter a conventional attack by Iran. If Iran’s threat went nuclear, America might have to extend its nuclear umbrella as well. Defense Secretary Robert Gates carefully stepped around that option last week while in Saudi Arabia and the United Arab Emirates, trying to reassure leaders who increasingly fear the prospect of an Iranian bomb.

Mr. Gates defended the sanctions strategy: “I think the prospects of success are certainly better than in a lot of other situations where sanctions have been applied,” he said. But he spent most of his time explaining the need for “defensive capabilities” against Iranian missiles.

Few doubt the missile threat can be contained. Strategists worry more that Iran might slip a crude weapon or nuclear material to terrorists, betting it couldn’t be traced back to Tehran. (It’s not a bad bet — the science of “nuclear attribution” is a lot weaker than it seems on “24.” )

Yet another argument against containment comes from Brent Scowcroft, the national security adviser under Presidents Gerald Ford and George H. W. Bush. “The ultimate consequence of trying to contain Iran has little to do with Iran itself,” he argues. “The biggest risk is that it will start an eruption of proliferation” around the Gulf, starting with Turkey, Saudi Arabia and Egypt. They would doubt the American deterrent capability, he said, and the problem would spread to Japan and South Korea.