THE NEGOTIATIONS with Iran due to begin this week in Istanbul may be the last chance for a peaceful settlement on its nuclear program, at least in a negative sense: If Tehran again refuses to make concessions, and continues to press ahead with uranium enrichment at a new underground facility, military action by Israel or the United States may become inevitable. Hardly anyone, however, thinks it likely that the United States and its five partners in the group negotiating with Iran will be able to strike a deal that ends the Iranian nuclear threat or satisfies United Nations resolutions on the issue. Supreme Leader Ali Khamenei appears firmly opposed to any lasting accord.

The pressing question, then, is whether some kind of partial bargain can be made that would break what looks like a slide toward war. Partly in public and partly in private briefings, the Obama administration has spelled out what it thinks is necessary: an Iranian agreement to cease its higher-level enrichment of uranium to the level of 20 percent; to export the some 100 kilograms of fuel already processed to that level; and to close down the new facility buried under a mountain near the city of Qom. While the administration hasn’t detailed what it would be prepared to give Iran in return, the minimum looks like a pledge to freeze further sanctions and possibly to repeal some of the harshest already adopted, such as those aimed at the Iranian central bank.

A deal along those lines would offend Israel and many in Congress. Like them, we have taken the position that Iran should stop all enrichment, as required by the U.N. resolutions, in order to obtain sanctions relief. But Iranian compliance with the administration’s terms could greatly reduce tensions. It would represent the first voluntary curb by Iran on its program since 2003, and it could prevent the program from moving into what Israel describes as a “zone of immunity,” in which it could be invulnerable to a conventional Israeli air attack.

For those who, like us, believe that military action against Iran is neither necessary nor wise in the coming months, a deal in which Iran met the administration’s terms would be a relief — but an unsatisfying one. It probably would prevent war. But the risk is that it would be counterproductive in the medium term, because it would ease what is now mounting economic pressure on Iran and allow the regime breathing space. It could leave the nuclear program in a stronger position than it was when the Obama administration began negotiations in the fall of 2009 — with more centrifuges and enough low-enriched uranium to make several nuclear bombs with further processing. If the regime refused a more comprehensive deal, or cheated, it might be difficult to restore sanctions that only now finally appear to be biting.

With the presidential election looming, President Obama might be happy to trade those problems for avoiding a major international crisis in the coming months. For us, the call is closer. But most likely the Iranians themselves will settle the matter. For better or for worse, the chances the regime will meet Mr. Obama’s terms don’t look good.