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.”…

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.”