IRAN HAS LAUNCHED a frantic flurry of initiatives in the past two weeks, most of them bellicose — from suspending oil shipments to Europe to allegedly attempting to assassinate Israeli diplomats. But the Obama administration and its allies appear likely to seize on Tehran’s contrasting dispatch of a 200-word letter agreeing to a renewal of talks on its nuclear program. Delivered at a moment when sanctions appear to be causing severe disruption to the Iranian economy, the offer raises the hope that the theory underlying years of U.S. policy — that international pressure would eventually cause Iran’s current leaders to bargain seriously — could at last be proved. It will, at least, be put to a crucial test.

The good news about the Feb. 14 letter from chief nuclear negotiator Sael Jalili is that it retreats from the ludicrously intransigent position that poisoned the last meeting between Iran and the “P5+1” nations — the five U.N. Security Council permanent members and Germany — in January 2011. Then Iran refused even to discuss its nuclear program. Now Mr. Jalili is saying that “talks for cooperation . . . on Iran’s nuclear issue could be commenced.”

The immediate question is whether Iran is using diplomacy — as it has several times before — as a way of buying time, even as it presses ahead with steps toward a bomb. Recent reports say that the nuclear program is close to passing another major milestone, with the startup of a uranium enrichment facility buried under a mountain near the city of Qom.

Fortunately, a test of Iran’s seriousness was underway this week as a delegation from the International Atomic Energy Agency visited the country. Its aim was to obtain agreement to a plan to answer outstanding questions about alleged work on weaponization, including interviews with scientists and a visit to a military base. On Tuesday night, the IAEA reported another Iranian failure to cooperate — which suggests that Supreme Leader Ayatollah Ali Khamenei has not changed his long-standing refusal to come to terms with the West.

In fact, it appears likely that Tehran perceives talks as an opportunity to undermine sanctions. Mr. Jalili’s letter referred to negotiations “based on step-by-step principles and reciprocity,” language that could describe a proposal originally put forward by Russia last year. Moscow outlined a sequence of steps in which Iran would receive relief from sanctions in exchange for incremental actions to satisfy the IAEA. Iran rejected the idea, but now the P5+1, urged on by the Obama administration, is discussing a modified version. Reportedly, it could grant some sanctions relief if Iran suspended only its higher-level enrichment of uranium, and surrendered material enriched to that 20 percent level.

Such a deal would be a retreat from Security Council resolutions that require Iran to cease all uranium enrichment and would ease the pressure on the leadership at precisely the wrong moment. Worried about that possibility, a bipartisan group of a dozen senators dispatched a letter to Mr. Obama last Friday opposing “any proposal that caps or limits sanctions” in exchange for “anything less than full, verifiable and sustained suspension of all enrichment activities.” If Iran is serious about a deal, it will meet the senators’ terms.