European diplomats said there was scope for an agreement on missiles and inspections, but not yet on the length of the deal. They argue that rewriting those terms would break the bargain they struck, not only with Iran but also with Russia and China, two other signatories. And breaching the deal, they say, would free Iran to pursue nuclear weapons again.
That is why, as the two sides prepare to meet again in Berlin next month, the Europeans are floating the concept of an add-on deal, which would extend rather than upend the existing deal. In Paris last week, they asked Mr. Hook to guarantee that if they agreed to an extension, Mr. Trump would promise not to tear up the accord on some other pretext.
Mr. Hook, a Republican lawyer who is the State Department’s director of policy planning, said he would bring the request back to Washington. European diplomats said they worry that Mr. Trump’s scorn for the deal runs so deep that he would find other reasons to pull out. Last month, he warned that the accord “is under continuous review, and our participation can be canceled by me as president at any time.”
Even if Mr. Trump did pledge to abide by the deal, it is far from clear that a successor deal would be endorsed by Russia or China, let alone the Iranians, who signaled in recent weeks that they are planning a new project — a fleet of nuclear-powered ships, fueled by Iranian-made reactors — that they say would justify resuming the production of nuclear fuel as the limits imposed by the deal expire over the next dozen years.
Still, the mere fact that the United States and Europe are trying to work out a compromise attests to the desire, on both sides, to find a solution that would satisfy Mr. Trump while not unraveling the deal.
The president’s national security team — Mr. Tillerson, Defense Secretary Jim Mattis, and the national security adviser, Lt. Gen. H. R. McMaster — has on three occasions talked him out of ripping up the deal. With each deadline to reimpose sanctions on Iran, that task gets harder.
There is an element of diplomatic legerdemain to the exercise, European diplomats acknowledge: How do you convince Mr. Trump that you have changed the deal without actually changing it?
“The supplemental deal is a diplomatic device that is being used to allow the Europeans to declare victory,” said Mark Dubowitz, a leading critic of the Iran deal who is nevertheless open to the idea.
“They can say they were able to keep the deal, remain steadfast to their commitment not to renegotiate it, but also to satisfy the U.S. and their own concerns that the length of the deal was too short,” said Mr. Dubowitz, the chief executive of the Foundation for Defense of Democracies.
By law, Mr. Trump must decide every 120 days whether to continue suspending longstanding American economic sanctions against Tehran, which the United States committed to lift as part of the deal. If he were to reimpose them, as he has threatened, that would effectively scuttle the deal.
Iran complains that Mr. Trump’s threats have already kept European banks from investing in major projects in Iran, denying it the benefits that were promised for giving up its enrichment program.
“If the same policy of confusion and uncertainties over the future of the deal continue,” Iran’s deputy foreign minister, Abbas Araqchi, who was a member of the negotiating team, said last week, “if companies and banks are not working with Iran, we cannot remain in a deal that has no benefit for us.”
The International Atomic Energy Agency said last week that it had received formal notice from Tehran about a “decision that has been taken to construct naval nuclear propulsion in future.”
Nothing in the 2015 agreement prohibits the construction of nuclear reactors to power ships and submarines, and those reactors would almost certainly not use nuclear-weapons-grade fuel. But they could provide a pretext for Iran to resume uranium enrichment.
The Europeans are most comfortable with enforcing new limits on Iran’s intercontinental ballistic missile development and testing. Missiles are not covered by the nuclear accord, but rather by a United Nations resolution, whose wording was negotiated by then-Secretary of State John Kerry just as the nuclear accord was coming together in Vienna in July 2015.
The State Department’s talking points said the United States viewed the nuclear and missile programs as “inseparable,” and said that “Iran’s development and testing of missiles should be subject to severe sanctions.” The Europeans have not gone that far, at least yet.
Inspections are potentially more problematic, given the administration’s rejection of arguments by the Islamic Revolutionary Guards Corps that all military-related sites are off limits to international inspectors.
But this has not been an issue yet: The inspections have been limited to declared nuclear sites, and the United Nations nuclear agency affirmed last week that it had been given all the access it sought. Inspectors have said they would try to inspect military sites only if they had intelligence suggesting that surreptitious nuclear activity was underway.
Once a deal is struck with the Europeans, administration officials intend to seek approval from Russia, China and Iran. But concerns that such a deal could not possibly pass muster with Iran will not deter them, a senior official said. Russia and China’s lack of participation would also matter little, given the stark consequences for Iran’s economy if Europe and the United States reimpose sanctions, this person said.
While American and European negotiators are working feverishly on a deal to preserve the 2015 accord, there are also preliminary discussions about what will happen if they fall short.
Reimposing sanctions, such as blacklisting the Central Bank of Iran, would cause serious problems for European companies, potentially precipitating a split in the trans-Atlantic alliance. But the president, Mr. Dubowitz noted, has the authority to phase in sanctions, which he could use as leverage to give negotiators more time to work out an agreement.
He could, for example, impose sanctions on the central bank for its support of the Islamic Revolutionary Guards Corps, which is accused of supporting terrorism, but waive the imposition of so-called secondary sanctions on foreign companies that do business with the bank until the next deadline in July.
For the Europeans, who are mostly satisfied with the deal and face no domestic political pressure to pull out, there is little joy in this exercise. But they feel they have no choice but to go along with it. One diplomat compared it to humoring an angry relative who controlled a family vacation estate, and periodically threatened to burn it down.