The UK, France, and Germany's decision to initiate the snapback mechanism under UN Security Council Resolution 2231 on 28 August represents a significant shift in the long and uncertain history of Iran's nuclear programme.
Snapback is a mechanism from UN Security Council Resolution 2231, which confirmed the nuclear deal with Iran (JCPOA) in 2015.
It states that if a party to the agreement believes Iran is in serious breach of its obligations, it can initiate a procedure that automatically reinstates all previous UN sanctions against Iran within 30 days - without the possibility of Russia, China or anyone else exercising their veto.
Europe is the latest to take this step, which rules out an extension of the 2015 nuclear deal and leads the international community into a new phase of relations with Tehran.
The sanctions that were suspended at the time are now being reinstated - without the possibility of a veto and a blockade in the Security Council.
The timing is no coincidence: Termination Day, the day on which all sanctions provisions would automatically expire, is scheduled for 18 October this year. The E3 had to act now or the resolution would have lost its effect.
Why now?
The European powers have decided to take this step after several years of failed negotiations.
Iran has significantly increased its stockpile of enriched uranium, restricted inspections by the International Atomic Energy Agency and severely limited transparency.
Attempts to find a compromise in Istanbul and Geneva have failed, and the political will in Tehran has clearly shifted in favour of a tougher line.
The Europeans have calculated that it is better to move now, while there is still a legal basis, than to be accused of having missed the last chance.
Reactions and division
Iran immediately rejected the move, calling it "null and void" and "illegal." The Iranian media used the opportunity to further mobilise the public by highlighting the history of European injustice and "economic violence."
In parallel, China and Russia openly supported Iran, rejecting the E3's decision and labelling it politically motivated.
Their support makes it clear that the reintroduction of sanctions will not have the same strength as in 2010, when the global community was much more united.
Consequences for Iran
This is not the first blow to the Iranian economy. In recent decades, Tehran has learnt to operate under restrictions.
The black oil market, networks of middlemen in Asia and Africa and direct cooperation with Russia and China have helped Iran maintain its main sources of revenue. The oil minister Mohsen Paknejad has said that Iran will continue to export, "regardless of any formal bans."
Trade costs will rise, transport and insurance will become more complex, and access to international finance will be harder
However, the reality is that trade costs will rise, transport and insurance will become more complex, and access to international finance will be harder.
For a country already struggling with inflation and unemployment, the return of sanctions means additional pressure on internal stability.
Tehran's political calculations
Iran perceives the snapback as proof that the West cannot be trusted. The hardliners in parliament and the Revolutionary Guard argue that the nuclear agreement was a "historic mistake" and that the country must now take the path of full independence, including the possibility of withdrawing from the Non-Proliferation Treaty.
For the regime, the sanctions will become an argument for further militarisation and the strengthening of alliances with Moscow and Beijing.
The government is relying on a mix of repression and nationalist rhetoric to keep power, even though the pressure on society will increase.
Prospects for Europe
For Europe, this step means the end of the illusion that the JCPOA can be saved.
The E3 has long tried to strike a balance between the US approach of "maximum pressure" and Iran's hardline policy.
Europe is practically joining the ranks of those who believe that Tehran only understands pressure
Now that the US has long since withdrawn from the agreement, Europe is practically joining the ranks of those who believe that Tehran only understands pressure.
Brussels and the most important European capitals are thus signalling that they are not prepared to wait until October to watch the agreement disappear without any legal support.
The broader geopolitical picture
The snapback comes at a time when the Middle East is already in flames - the war in Gaza, the instability in Iraq and Syria, the Israeli-Iranian confrontation...
The return of sanctions against Iran will increase Tehran's sense of isolation and probably increase its willingness to act more aggressively in the region through proxies.
Greater support for the Houthi rebels in Yemen, closer ties with Lebanon's Hezbollah and greater cooperation with Russian operations in Syria seem the most likely outcomes.
Protests are not impossible, but the experience of recent years shows that the regime has enough mechanisms at its disposal to suppress them - Tehran
This will not be presented as a response to the Europeans but as part of a broader "Axis of Resistance" strategy.
In the coming months, Iran will try to show that the sanctions cannot destroy its economy.
In practice, the consequences will be seen in the form of lower foreign currency inflows, difficulties in trade and new waves of discontent.
Protests are not impossible, but the experience of recent years shows that the regime has enough mechanisms at its disposal to suppress them.
On the other hand, Europe will try to show that it still has influence on the global security architecture, even if the role of the US remains crucial.
Russia and China will have the opportunity to bind Iran even more closely to their economic and security projects.
For the West, the snapback is an attempt to show resolve and avoid a scenario in which Iran becomes a nuclear power without serious opposition from the international community.
Iran will present this as proof that agreements with the West have no lasting value and that only independence and alternative alliances can ensure survival.
The result will not be another frozen cycle of negotiations, but a decision on whether Iran will use the sanctions as a pretext to accelerate its move towards nuclear weapons or whether it will have to give up some of its ambitions under pressure.
In either case, the snapback is the trigger that moves the Iranian issue from the realm of agreements and resolutions into the realm of open strategic competition between the major powers.
