The most dangerous implication of the seizure of the 15 British sailors and Royal Marines — other than the direct threat to them — is that Iran is even more unpredictable and confrontational than it had seemed.

The second is that Russia, despite its own now-explicit unease about Iran’s nuclear ambitions, is a serious obstacle to Britain’s efforts to rally support.

When Iran seized the sailors, it was reasonable to hope that this might be one of the impetuous accidents to which this complicated regime are prone, their factions often disorganised or contradictory. That hope determined Britain’s initially low-key response; it was plausible that an over enterprising group within the Islamic Revolutionary Guards Corps might have decided to seize the sailors, leaving government officials, scrambling into their offices during the New Year holiday, to fit a framework of policy onto some awkward freelancing.

That hope has gone. If officials found the original predicament embarrassing, they have chosen to escalate it, particularly with the letter from the servicewoman Faye Turner calling on Tony Blair to pull out of Iraq.

Why now? The clash over Iran’s nuclear programme is one reason. Britain, along with the US and France, has been pushing in the UN Security Council for tough sanctions against Iran for its failure to reassure the world that its efforts to develop nuclear power do not conceal a weapons programme. The recent imposition of some sanctions had already ratcheted up tension.

British officials have been encouraged that in the nuclear talks, even since the election of the hardliner President Ahmadinejad two years ago, Iran has behaved like a “rational regime” which wants to avoid sanctions and calculates the penalties and rewards of its actions. It appeared shaken by the united front of the Security Council last year, while the US’s own financial sanctions have appeared to have real and rapid effect.

Most of all, Tehran has appeared perturbed by the sudden coolness of Russia, which has withheld shipments of fuel from Iran’s first reactor because of concerns about the ultimate intentions. Yet Russia’s support for the Western position on the nuclear issue has not extended to the hostage sailor crisis — it forced the council to water down the tough condemnation that Britain wanted.

No Western diplomat can have taken comfort from Iran’s escalation of the nuclear conflict. It has threatened to stop cooperating with the International Atomic Energy Agency; that cooperation is the basis for its claim to be behaving legally under the Nuclear NonProliferation Treaty, but has also given other countries their only windows onto Iran’s nuclear sites.

Iraq is another probable cause of the seizure of the sailors. Iran has condemned the US “surge” of troops and the US seizure of Iranians claimed to be military agents. Still, the US is not Britain; it cannot have been lost on Tehran that Mr Blair has said he will soon bring back about a third of British forces in Iraq. To hold hostage the troops of a country that says it is heading for the exit counts as picking a fight.

A letter from Tehran, said to be asking for Britain’s commitment never to enter Iranian waters again, might be the first sketch of an exit from the crisis that Tehran would consider face-saving. But now that the stand-off has gone into its second week, one can only conclude that the regime in Iran, in its various conflicts with the West, is behaving in a more systematically confrontational way than those who negotiate with it had hoped.

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