After a resounding defeat in the U.N. Security Council, the United States is poised to call for the United Nations to reimpose sanctions on Iran under a rarely used diplomatic maneuver – a move that is likely to further isolate the Trump administration and may set off a credibility crisis for the United Nations.

The sanctions had been eased under the 2015 nuclear deal that President Donald Trump withdrew from two years ago. But last week the U.S. lost its long-shot bid to indefinitely extend an international arms embargo on Iran and has now moved to a new diplomatic line of attack.

U.S. Secretary of State Mike Pompeo is set to travel to New York on Thursday to notify the Security Council president that the United States is invoking the "snapback" mechanism in the council’s resolution that endorsed the nuclear deal. It allows participants to demand the restoration of all U.N. sanctions in a complicated procedure that cannot be blocked by a veto.

The State Department is expected to announce Pompeo’s travel plans Wednesday, but he and Trump have made no secret of their intention to invoke snapback, especially since their attempt to extend the arms embargo suffered an embarrassing defeat last Friday. The U.S. won just one other "yes" vote, with China and Russia opposed and the 11 other members abstaining.

Just like the arms embargo extension, the administration’s snapback plan is bitterly opposed by China and Russia as well as the other Security Council members, including U.S. allies Britain and France, and could set the stage for a battle over the legitimacy of the U.N.'s most powerful body.

Alone among the council’s 15 members, the U.S. argues that as an original participant in the nuclear deal it retains the right to demand the restoration of sanctions. The others, which still support the deal, maintain the U.S. lost that standing when Trump pulled out of the accord in 2018, but it isn’t clear if they can stop the invocation of snapback through technical procedural means.

The U.S. argument is highly controversial. It has been ridiculed by the Chinese, Russians and Europeans, and not even the biggest Iran hawks in the United States all agree with it.

Former Trump national security adviser John Bolton, no slouch when it comes to anti-Iran positions, has long said that the U.S. lost its snapback standing when it withdrew from the deal and that moving ahead is not worth the damage it could do to U.S. veto power in the council.

In a stunningly rare moment of agreement, Iran’s Foreign Minister Mohammed Javad Zarif praised Bolton this week. "At least he is consistent – a trait notably absent in this U.S. administration," Zarif tweeted.

And, former U.S. Undersecretary of State for Political Affairs Wendy Sherman, a top Obama administration negotiator of the nuclear agreement, said: "It was never expected that someone who withdrew from the (deal) would have standing to in fact bring the snapback provision."

Thus, the administration’s insistence on moving ahead has set the stage for a contentious dispute and the possibility that the U.S. call would simply be ignored by other U.N. members. That outcome would potentially call into question the Security Council’s ability to enforce its own legally binding decisions.

Under the terms of Security Council Resolution 2231, which enshrined the nuclear deal and to which the U.S. remains a party, the invocation of snapback for significant Iranian noncompliance starts a 30-day clock during which the council must vote affirmatively to continue the sanctions relief that Iran was given in return for curbs on its nuclear program.

Pompeo is expected to present evidence of significant noncompliance, likely the latest reports of the U.N. nuclear watchdog, the International Atomic Energy Agency (IAEA), on Thursday. Iran does not deny violating some terms of the deal but says its actions have been forced by the U.S. withdrawal and the Trump administration’s reimposition of U.S. sanctions.

As envisioned by the Obama administration, which led the negotiations that culminated in the nuclear deal, the United States or any other permanent member of the council could use its veto to block the continuation of sanctions relief. In theory, that would result in the reimposition of sanctions.

But whether any other council member will respond to the U.S. move by introducing a resolution to extend sanctions relief is an open question. Some U.N. experts believe the others will just ignore the Americans, leaving the Trump administration in the possible position of having to introduce its own resolution to extend sanctions relief for the sole purpose of vetoing it.

"We don’t know if any country will do that," said Richard Gowan, the U.N. director of the International Crisis Group. "If the general view of the council is that the U.S. doesn’t have standing, it’s quite possible that no council member will even engage at that level."

"The U.S. could actually table a resolution of its own and then veto it, just to show that it is going through the procedural motions, although that would look a little bit farcical," Gowan said.