By Frances Harrison
BBC News, Tehran
This is not the first time Iran has seized British sailors for allegedly intruding into its territorial waters - the same thing happened to eight UK servicemen in 2004.
Ayatollah Khamenei made a defiant start to the Persian new year
But this time there are reasons why it could be more serious.
The incident comes just two days after Iran's Supreme Leader Ayatollah Ali Khamenei gave an unusually aggressive speech to mark the new Persian year.
He said: "In case the enemies of Iran intend to use force and violence and act illegally, without a doubt the Iranian nation and officials will use all their capabilities to strike the invading enemies."
It was an oddly defiant and hostile tone to strike for a new year speech.
One commentator, Sayeed Laylaz, has drawn a parallel with President George W Bush's state of the nation address in January, which was followed immediately by a US attack on an Iranian office in Irbil in northern Iraq and the seizure of five Iranians who are still being held by the US.
Mr Laylaz points out that the speech of Mr Khamenei was swiftly followed by the capture of the British sailors.
So far Iran has not indicated what political capital they plan to make out of the captured sailors
Then there is the timing of the capture of the sailors, the day before a key vote in the UN Security Council on imposing a second raft of sanctions on Iran over its nuclear programme.
Iranian political scientists say there are factions in the Revolutionary Guards who are spoiling for a fight - extreme hardliners who think if a confrontation with the West is inevitable it is better it happen over the nuclear issue than Iran's human rights record.
There has also been more international condemnation this time, whereas in 2004 it was treated as a bilateral issue between Britain and Iran.
Some in Iran argue that the authorities have much less to lose from aggressive behaviour now, with no negotiations currently underway on the nuclear issue.
Sayeed Laylaz compares Iran to a cornered cat that has no option but to strike back.
Global concern is already high over Iran's nuclear programme
The argument is that Tehran is so isolated internationally that it needs an asset - a card to play to force the West to engage.
If this is the thinking it might explain why it is British military forces who've been seized and not Americans, because the British still have diplomatic relations with Tehran - unlike the US.
So far the Iranian authorities have not indicated clearly their intentions, what if any political capital they plan to make out of the captured sailors.
The worry is that the situation will worsen considerably if Iran links the fate of the British to that of the five Iranians held by the Americans in Iraq.
The US says those five men are elite Revolutionary Guards up to no good in Iraq. Tehran says they are diplomats.
The situation could complicated further if a clear link is made to UN action against Iran's nuclear programme.