Last week's feeding of fuel into Iran's Bushehr nuclear reactor came as a surprise. For years, Russia dragged its feet and found recurrent excuses to delay the completion of the Bushehr project. The ceremony marking the start of the reactor last Saturday indicates the last obstacle has now been removed.
Why Russia decided to proceed is open to debate - though one can surmise that Moscow wished to register its discontent with the autonomous sanctions that the US, Europe and other Western countries recently adopted against Iran's energy sector.
So far, Western reactions have been muted - despite the fact that Russia's decision runs contrary to the spirit of the resolution set out in the latest sanctions, which was adopted by the UN Security Council with Russian approval.
Most crucially, the activation of the Bushehr plant spectacularly breaks Iran's international isolation.
What does it mean, though, for Iran's nuclear programme? The agreement regulating the construction and functioning of Bushehr stipulates that Russia will supply its fuel and that once the fuel is spent, Russia will collect it and return it to Russia. Meanwhile, the reactor is under international monitoring and safeguards.
Highlighting these terms, Moscow argues, not unreasonably, that Bushehr has no relevance for Iran's alleged quest for nuclear weapons since Russian guarantees ensure no risk of proliferation.
This is also the view of the Obama administration and the British government, which officially responded to the development by noting how Bushehr proves that Iran does not need to enrich uranium indigenously - since the nuclear fuel for its only functioning nuclear power plant is being supplied from outside the country.
Why then so much concern, including the claim, voiced last week by former US Ambassador to the UN, John Bolton, that Israel had only days left to bomb Iran?
Bushehr lends itself to non-civilian purposes in two ways. First, Iran's nuclear scientists will acquire much-needed knowledge and experience from its operation which until now they lacked.
And secondly, if Iran decided to do so, it could extract plutonium from the spent fuel.
Admittedly, Iran would have to introduce important changes to Bushehr and its violation of the terms of agrement with Russia would be quickly detected.
But regardless, if Iran made that decision, it could weather the resulting political storm and offset the damage by reaping the benefits of plutonium extraction. According to experts, Bushehr, once reconfigured, could produce up to 250 kilograms of plutonium per year, enough for 30 nuclear bombs.
The risk, then, is that once the reactor is activated, if Iran decided to renege on its commitments, it would be too late to do anything about it.
A military attack would cause a Chernobyl-type incident, with thousands of potential casualties and contamination, and the added damage of a nuclear disaster in the oil-rich Persian Gulf.
Whatever Iran's intentions and Russia's ability to enforce its agreement with Tehran, the reactor's activation shields it from a pre-emptive military attack.
Only Iran's goodwill and solemn commitment to its agreements with Russia stand in the way
of Tehran being able to use a civilian reactor for ulterior motives.
Emanuele Ottolenghi is a Senior Fellow at the Foundation for Defence of Democracies in Washington