By Paul Reynolds
World affairs correspondent, BBC News website
Many in Iran are deeply suspicious of the British.
The mutual expulsion of diplomats by Iran and Britain shows that relations between these two old antagonists are alive and bad.
The expulsions started on the Iranian side when two British diplomats were expelled with the usual claim of "activities incompatible with their status".
This often means spying but it could mean anything and the Iranians did not explain. More out of routine than anger, Britain retaliated in kind.
Subsequently Iran detained a number of locally-engaged Iranian staff at the embassy, accusing them of taking part in the demonstrations. Britain protested.
It all followed a speech by Supreme Leader Ayatollah Khamenei, who called Britain the most "evil" of the "hungry wolves in ambush" on Iran.
The expulsions tell us quite a bit about the Iranian government - how paranoiac it remains and how ready to blame foreigners for its troubles.
But why has it singled out Britain?
There is a history to this.
"There is a deep-rooted belief in Iran that Britain is always up to something, is never passive and always devious," said Rosemary Hollis, Middle East analyst at City University in London.
Ayatollah Khamenei has recently been highly critical of the UK.
"I meet it all the time with Iranians. It is a combination of history and current British involvement with Iran.
"One issue is the setting up of the BBC Persian TV channel.
"Another is the presence in the UK of the Iranian opposition group MKO."
The MKO is the People's Mujahedin Organisation, which was taken off the list of terrorist groups by the EU in January.
"But it is also possible that the Iranians are labelling the Brits as meddlers in order to avoid attacking the United States and to leave the door open to Obama," Ms Hollis adds.
US President Barack Obama has made an offer of talks with Iran to which Iran has not yet formally responded.
As for the history, it depends on how far back you want to go.
You could go back to 1813 and the Treaty of Gulistan, under which Persia was forced to concede territory to Russia. The treaty was put together by British diplomat Sir Gore Ouseley and is regarded as a humiliation in Iran.
The myth - or reality - of the devious British was established.
Britain was also instrumental in setting Iran's borders with India in the 1860s.
Then in the 1920s, British forces in Iran under General Edmund Ironside (later British land forces commander in World War II after Dunkirk) helped put Reza Shah on the Peacock throne. His son was Mohammed Reza Pahlavi, the Shah overthrown in the Islamic revolution of 1979, so there is a direct link back to British actions decades ago.
In more modern times, the event that really led to the mistrust of Britain - and the US - was the coup against the elected government of Mohammed Mossadeq in 1953.
It is ironic that in the current crisis, the British government tried to keep a low profile, not wanting its 'historical baggage' ... to be used as an excuse by Iran to blame it for interference
Mossadeq had wanted to nationalise the Anglo-Iranian Oil Company in which the British had a majority share. The British and Americans organised a coup, put Mossadeq under house arrest and placed Pahlavi firmly in control as Shah.
After the Shah himself was removed, the Islamic revolutionaries turned their attention more to the "Great Satan", the US, than the UK. Hostages were taken at the US embassy and President Carter launched a disastrous operation to try to free them. There followed many barren years.
There have been brief rapprochements now and then. An alliance of convenience between the Reagan administration and Iran saw the US get arms to Iran in exchange for the freeing of western hostages in Lebanon.
During the Iran-Iraq war, which the Iraqi leader Saddam Hussein started, western support for Iraq was deeply resented in Iran.
Business as usual
More recently, Iran released 15 Royal Navy personnel after seizing them in the Gulf. But that turned out to be a gesture not a new policy of friendship.
The British Museum even tried to do its bit with an exhibition this year about the reign of Shah Abbas in the 16th and 17th Centuries, and for which it got the cooperation of the Iranian authorities.
But nothing has ever really been resolved and the antagonism brought about by suspicions surrounding Iran's nuclear programme and the sanctions imposed by the Security Council (pressed for by the US and UK) only made relations more tense.
It is ironic that in the current crisis, the British government tried to keep a low profile, not wanting its "historical baggage", as one official put it, to be used as an excuse by Iran to blame it for interference.
This has happened anyway. It is business as usual.