Following months of wrangling, the UN Security Council has agreed to impose sanctions on Iran for its refusal to suspend a nuclear energy programme that Tehran insists is purely for civilian use and that others fear masks an intention to develop nuclear weapons. The formal discussion was brief, the speeches were to the point, and the vote was unanimous. Neither Russia nor China applied a veto.

The price of unanimity, of course, was the strength of the resolution. The proposed travel ban has been watered down, as has the proposed freeze on private assets. A monitoring committee provided for by the resolution will have more discretion than Iran's fiercest critics would have liked. And Russia will be able to continue the nuclear energy co-operation programme it was keen to protect.

But the central demand - that Iran halt its uranium-enrichment programme and all nuclear research that might be judged to have a military purpose - remains in place. So does the ban on imports and exports of certain sensitive materials. And this is the point. The painstaking efforts that the Europeans have made to persuade Iran to comply with the demands of the international nuclear watchdog now have the formal backing of the UN Security Council.

This will not stop Iran's defiance. Tehran quickly made clear its contempt. In domestic political terms, President Ahmadinejad could hardly have responded otherwise, especially after his party's recent electoral setback. But this resolution puts down a marker - and that marker applies not only to Iran.

For perhaps just as significant as the signal sent to Tehran are the more general messages conveyed by the negotiation process and its successful outcome. The first is that unanimity requires give and take and that there are times when the route to enforcement will - and should - be slow and circumspect. The second is that the Security Council has learnt the lesson of the infamous resolution 1551 on Iraq and its fatal ambiguities. Russia's ambassador spelt out that there was no way this resolution could be read as authorising the use of military force.

The third - and perhaps most hopeful - is that the United States conducted itself as just another member of the Security Council rather than a country that, alone, could dictate the outcome. The irascible John Bolton was not in evidence following his recent resignation, nor - more to the point - were his attitudes. Weakened by Iraq, the US had to choose between accepting concessions, and belligerence from the sidelines. It chose concessions. This adjustment in Washington could, just, open the way for a similar shift in Tehran. That, at least, is what we must hope.