European leaders and diplomats from Germany, Britain, France and the European Union have repeatedly insisted that the nuclear deal is working, that Iran is complying with the accord and that no renegotiation is possible. Russia and China, which also signed the agreement, have evinced no interest in a renegotiation. And Iran’s foreign minister, Mohammad Javad Zarif, has warned that Iran would insist on new concessions, such as a return of its nuclear fuel, in any new agreement.

The quiet struggle between the Trump administration and its most important European allies has been the focus of much of the jockeying after President Trump decided this month not to certify that Iran was complying with all the terms of the agreement. That kicked to Congress the decision about whether to reimpose sanctions and abandon the accord.

Allies of the administration have argued that should Congress decide to reimpose sanctions on Iran, European companies would have little choice but to comply since they would not want to risk being shut out of the $19 trillion American market in favor of Iran’s $400 billion one.

With the Europeans on board, Iran would feel the sting of significant sanctions even if Russia and China did not join in. Without the Europeans, however, Iran would feel few effects from sanctions by the United States — the two countries have little trade — while potentially being able to pursue nuclear weapons with the nuclear accord ended.

European diplomats have said they would defend their companies against such sanctions, potentially setting up an epic battle between close allies and two of the largest commercial markets on the planet.