Britain, France, and Germany -- the so-called E3 -- have warned they will trigger an oft-threatened but never-used mechanism at the United Nations that could reimpose global sanctions on Iran. Their deadline: the end of August, unless Tehran makes tangible progress in nuclear talks with the United States.

The "snapback" mechanism is a special process created alongside the 2015 Iran nuclear deal, formally endorsed by UN Security Council Resolution 2231. Its purpose is to ensure that if Iran seriously violates the deal, the international community can swiftly restore the full set of UN sanctions that existed before the agreement without getting bogged down in great-power vetoes or endless negotiations.

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Although "snapback" is a nickname rather than a term in the legal text, it has become the common shorthand for the automatic return of sanctions. The E3 have already notified the UN that they are prepared to use it, a step that could dramatically escalate tensions over Iran's nuclear program. The mechanism itself is due to expire on October 18, giving the Europeans only a narrow window to act.

How Do UN Sanctions Snap Back?

The nuclear deal, formally known as the Joint Comprehensive Plan of Action (JCPOA), includes a dispute process. If a participant in the deal, such as the E3 states, believes Iran is in "significant non-performance," they can raise the issue through the JCPOA's built-in mechanisms and ultimately refer it to the UN Security Council.

Once the issue reaches the UNSC, a 30-day clock starts. During those 30 days, the council would need to adopt a new resolution to continue sanctions relief for Iran. If that resolution does not pass in time, the old UN sanctions that were lifted under the 2015 deal automatically come back into force. No further vote is required.

Iranian Foreign Minister Abbas Araqchi has insisted for weeks that the E3's legal authority to trigger the mechanism is "highly questionable" because they demand Iran abandon uranium enrichment -- which he argues runs contrary to the JCPOA.

Their argument is that the E3 are breaking the deal, and therefore no longer parties to it.

But "there is no merit to this argument," said Richard Nephew, who served as the lead sanctions expert for the US team that negotiated the JCPOA.

"There is no mechanism by which another JCPOA participant can kick a JCPOA participant out of the agreement," he told RFE/RL. "The E3 never withdrew like the United States, so they maintain their rights."

Iran has alsothreatenedto respond, including by withdrawing from the nuclear Non-Proliferation Treaty (NPT).

Iranian diplomats will meet their European counterparts on August 28 in a last-ditch attempt to prevent the mechanism from being triggered.

Why It's Called 'Snapback'

A key feature -- and the reason it's called "snapback" -- is that the process cannot be blocked by a veto. Normally, one of the five permanent UNSC members could veto a resolution to impose sanctions. Here, the system is reversed: To keep sanctions relief in place, the council must pass a new resolution.

Any veto of that resolution prevents it from passing, and because the default is that sanctions return if no resolution is adopted within 30 days, a veto actually speeds the snapback rather than stopping it. In practice, once a participant triggers the process, it is very difficult to prevent the old sanctions from coming back.

Top diplomats of JCPOA parties prepare for a group photo after clinching a landmark nuclear deal in Vienna on July 14, 2015.

Iran says it is working with China and Russia -- both permanent members of the UNSC and parties to the JCPOA -- to "stop" the process. That, Nephew says, is not legally possible.

He noted that the only argument Beijing and Moscow can make is that the nuclear deal no longer exists, "but, that's really, really hard to do."

What Sanctions Will Return?

If the snapback process is successful, the UN will restore the six Iran-related Security Council resolutions adopted between 2006 and 2010.

These include wide-ranging restrictions such as a UN conventional arms embargo, measures curbing activities related to ballistic missiles capable of delivering nuclear weapons, and an array of asset freezes, travel bans, and other proliferation-related rules.

In practical terms, this means reinstating a broad, UN-backed legal framework that governments, banks, shippers, insurers, and technology providers take seriously.

Will This Affect Iran Much?

Iranian officials have been downplaying the potential impact of renewed UN sanctions, claiming the effects are exaggerated.

Even though the United States already maintains extensive unilateral sanctions that heavily constrain Iran's economy and energy exports, renewed UN sanctions would still matter.

Nephew argued that Iran's nuclear and missile trade would be hit hardest. "Both of those things will be illegal, along with the dual use imports that Iran will need to rebuild these programs," he said.

Iran's insistence that the sanctions won't hurt much contrasts with its warnings of retaliation if they are reimposed.

"The sanctions cannot be so bad that they'd withdraw from the NPT and so meaningless as to not matter," Nephew said.