WASHINGTON — As the Obama administration and its European allies toughened economic sanctions against Iran on Monday — blocking its access to the world financial system and undermining its critical oil and gas industry — officials on both sides of the Atlantic acknowledge that their last-ditch effort has only a limited chance of persuading Tehran to abandon what the West fears is its pursuit of nuclear weapons.

That leaves open this critical question: And then what?

While the United States and Israel have not taken military options off the table, pursuing them is unpalatable, at least for now. Several American and European officials say privately that the most attainable outcome for the West could be for Iran to maintain the knowledge and technology necessary to build a nuclear weapon while stopping short of doing so. That would allow it to assert its sovereignty and save face after years of diplomatic tensions.

While that might seem to be a big concession on the part of the United States, Iran would first have to make even bigger ones: demonstrate that it could be trusted and drop its veil of secrecy so that inspectors could verify that its nuclear work was peaceful, steps Iran has resisted.

In other words, Iran would have to become a country like Japan, which has the capability to become an atomic power virtually overnight, if need be, but has rejected taking the final steps to possessing nuclear weapons. “If you’re asking whether we would be satisfied with Iran becoming Japan, then the answer is a qualified yes,” a senior European diplomat said. “But it would have to be verifiable, and we are a long ways away from trusting the regime.”