The “snapback” mechanism was the enforcement procedure built into the legal framework of the 2015 Iran nuclear deal. Its purpose was to ensure that, if Iran was considered to be in serious breach of its commitments, the previous United Nations sanctions could be restored without the need for a new sanctions resolution. This made it an unusual legal device, because it reversed the normal logic of Security Council voting. Instead of requiring a positive vote to reimpose sanctions, the system required a positive vote to keep sanctions relief in place. If that vote did not happen, the old sanctions automatically returned.
The legal basis of this mechanism was UN Security Council Resolution 2231, which endorsed the JCPOA and reorganized the earlier sanctions regime against Iran. Under this framework, if a participant state claimed that Iran was in “significant non-performance,” it could notify the Security Council. That notification triggered a 30-day period. During those 30 days, the Council could adopt a resolution to continue the lifting of sanctions. If no such resolution was adopted within that period, the sanctions that had existed before 2015 automatically came back into force. The central legal feature of snapback, therefore, was automaticity: the reimposition of sanctions followed from the combination of notification and Council inaction.
This structure had major institutional consequences. In ordinary Security Council practice, a veto is often used to block coercive measures. In the snapback system, however, the procedural burden was reversed. Because the relevant resolution was one to continue sanctions relief, any permanent member could prevent that outcome by vetoing it, or the Council could simply fail to adopt it. In either case, the result was the same: the restoration of the prior sanctions regime. This is why snapback was often described as a mechanism specifically designed to make the return of sanctions easier than the preservation of relief.
For several years, the mechanism remained legally available but was not activated. That changed in 2025, when the E3 states — France, Germany, and the United Kingdom — initiated the procedure on the ground that Iran was in significant non-performance of its obligations. Their notification started the 30-day process provided for in Resolution 2231. Because no Security Council resolution was adopted within that period to preserve the sanctions-lifting framework, the previous UN sanctions were restored. The reactivation of snapback therefore marked the point at which a dormant enforcement clause became operational and the sanctions-relief structure established in 2015 effectively came to an end.
In substance, the reactivation of snapback meant that the legal order created by Resolution 2231 shifted back toward the pre-JCPOA sanctions model. What had originally been conceived as a safeguard against Iranian non-compliance was transformed into an operative mechanism for restoring coercive measures at the international level. The importance of this development lies not only in the return of sanctions as such, but in the way international law was structured to produce that result: not through a new collective decision to punish, but through a procedural design in which the absence of a decision was itself legally decisive.
In summary, snapback was a mechanism under the JCPOA and Resolution 2231 that allowed the automatic restoration of earlier UN sanctions if Iran was alleged to be in serious breach and the Security Council did not act within 30 days to maintain sanctions relief. Its reactivation in 2025 was significant because it converted a latent enforcement provision into a concrete legal outcome, bringing the earlier sanctions regime back into force and ending the operative effect of the UN sanctions-relief framework established in 2015.
It was reactivated on 28 August 2025, when France, Germany, and the United Kingdom formally notified the UN Security Council that Iran was in “significant non-performance,” thereby triggering the snapback procedure.
The trigger date was 28 August 2025, while the restored UN sanctions took effect after the 30-day period, at the end of September 2025.
The EU then formally reimposed the corresponding EU measures on 29 September 2025.