What is it?
The trigger mechanism, or snapback, is a provision in UN Security Council Resolution 2231, which endorsed the 2015 nuclear deal (JCPOA, known in Persian by the acronym Barjam). It allows the UN sanctions lifted under the deal to be reimposed on Iran. Its Persian name is mekânizm-e mâshe, and mâshe means "trigger."
How does it work?
The mechanism reverses the usual logic of the veto. A party to the deal notifies the Security Council that Iran is in significant breach of its commitments, which starts a 30-day clock. If the Council doesn't adopt a resolution to keep the sanctions lifted within that period, the old UN sanctions return automatically. No new sanctions resolution is needed. Because continued relief requires a resolution, a veto by a permanent member can't stop the sanctions from returning. It guarantees their return instead.
According to the Arms Control Association, this authority was due to expire on October 18, 2025, ten years after the deal's adoption.
Why does it matter?
France, Germany and the United Kingdom (the E3) triggered the mechanism on August 28, 2025. According to a Mayer Brown summary, UN sanctions came back into force on September 27, 2025, at 8 p.m. U.S. Eastern time. The European Union followed by reinstating its own sanctions on September 29–30, 2025. The restored measures include an arms embargo, bans on sensitive nuclear and missile activities, and asset freezes and travel bans on designated people and entities.
Snapback is widely seen as a turning point for Iran's economy and diplomacy. Since then, the rial's slide, restrictions on companies that trade with Iran and the nuclear talks have all been debated on a new legal footing. The Persian press regularly revisits the mechanism's anniversary and its consequences.
Debates
The mechanism's legal validity is disputed. According to its Wikipedia entry, Russia and China rejected the reimposition as legally void. Iran also argues that the E3 failed to meet their own obligations after the U.S. withdrew from the deal in 2018, and so had no right to trigger the mechanism. The E3, for their part, say Iran built up a stockpile of highly enriched uranium with no credible civilian justification. This split also affects how UN members enforce the sanctions.
