So what, exactly, does President Obama plan to do if, as everyone expects, these sanctions fail, just as the previous three did?

There is a Plan B — actually, a Plan B, C, and D — parts of which are already unfolding across the Persian Gulf. The administration does not talk about them much, at least publicly, but they include old-style military containment and an operation known informally at the C.I.A. as the Braindrain Project to lure away Iran’s nuclear talent. By all accounts, Mr. Obama has ramped up a Bush-era covert program to undermine Iran’s nuclear weapons infrastructure, and he has made quiet diplomatic use of Israel’s lurking threat to take military action if diplomacy and pressure fail.

But ask the designers and executors of these programs what they all add up to, and the answer inevitably boils down to “not enough.” Taken together, officials say, they may slow Iran’s progress toward a nuclear weapon, which has already run into far greater technical slowdowns than anyone expected. If the pressure builds, Iran might be driven to the negotiating table, which it has avoided since Mr. Obama came to office offering “engagement.”

But even Mr. Obama, in his more-in-sadness-than-anger description on Wednesday of why diplomacy has so far yielded nothing, conceded “we know that the Iranian government will not change its behavior overnight” and went on to describe how instead the sanctions would create “growing costs.”

That assessment sounds like the now-familiar combination of pragmatism and patience that Mr. Obama has tried to make the hallmark of his approach to foreign policy. But in the case of Iran, he is running up against ticking clocks. As Mr. Obama noted in April, once Iran passes a certain point, it may be impossible to know when it has taken the last steps to manufacture a weapon.

On Thursday, Iran responded to the new sanctions by threatening to further cloak its nuclear program from international oversight by revising its relationship with the United Nations’ nuclear watchdog. State-run Press TV quoted Alaeddin Boroujerdi, the head of national security and foreign policy in the Iranian Parliament, as saying that legislators would meet Sunday to “push for legislation to reduce” Iran’s relations with the International Atomic Energy Agency.

Some top officials in the Pentagon and the intelligence agencies say they wonder whether the White House has truly grappled with the question of how far Iran can be permitted to go, and what kind of risks Mr. Obama is willing to take beyond sanctions.

“It’s not the kind of question you win many points asking,” said one senior official who participated in many of the debates over Mr. Obama’s options, “because once you draw a line in the sand, you have to decide how you are going to act when the Iranians step over it.”

The need to confront those decisions appeared to be the underlying message of a secret memorandum Defense Secretary Robert M. Gates sent to the White House in January, in which he posed questions about tactics and strategy presupposing that the fourth round of sanctions would be insufficient. Though Mr. Gates said later that the memorandum was a routine effort to look ahead, some officials interpreted it as an effort to force a debate on the hardest questions.

Sanctions are a tempting tool for any president. They impose more pain than doing nothing or issuing ritual diplomatic condemnations, and they stop well short of military confrontation. Unfortunately, when it comes to stopping countries from getting the bomb, history suggests they are rarely effective.

Washington swore for years it would stop India and Pakistan from joining the nuclear club and briefly turned off aid to them. Today it works secretly with Pakistan to secure its arsenal and has signed a treaty with India permitting it to buy nuclear material.

North Korea has been under sanctions for years and is broke to boot; that did not stop it from conducting two crude nuclear tests. While some countries have been persuaded to give up their weapons or weapons dreams — South Africa, Libya, South Korea among them — the conditions were radically different than they are in the case of Iran.

“The sanctions as configured now are not going to have any appreciable impact on Iran,” said Rolf Mowatt-Larssen, who tracked nuclear programs around the world for the C.I.A. and the Energy Department before moving to Harvard two years ago. “It’s not going to do it. And the reality is that there isn’t a more viable military option.”

Mr. Obama’s aides say they know sanctions are a limited tool and that military options are the last and the riskiest choices, so they have reached for others.

American-controlled antimissile systems have been quietly placed in Arab states around the Persian Gulf. This is classic containment, but it is of little use against the nuclear program. While Iran has a growing conventional missile arsenal, intelligence experts believe it will be years before it could make a nuclear weapon that could fit atop a missile. Their fear instead is a weapon that could be handed off to Hamas or Hezbollah in a truck, a threat against which the antimissile systems are of no use.

The administration has continued to support Iran’s opposition groups, but treading carefully for fear of appearing to meddle in internal Iranian politics. On Thursday, Senator John McCain argued anew for regime change, but he was careful to say it had to be “peaceful change, chosen by and led by the people of Iran.” That is the kind of change whose timing no White House can control.

The Braindrain program has lured defectors out of the country, sometimes with laptops full of data about Iran’s progress. One of the most recent defectors showed up on YouTube in recent days, first claiming that he had been kidnapped, then that he was simply a student studying in the United States. There is little doubt he was part of the program, but many questions remain about how much he knew and how it could help the United States. “The big effect is psychological,” one former intelligence official said. “It tells the Iranians we are inside their program.”

So does the covert effort to make equipment fail, which is believed to have had some successes. But, like sanctions, this effort is unlikely to do more than delay the day of reckoning, unless Mr. Obama gets lucky.

Alan Cowell contributed reporting from Paris.