By Roger Hardy
BBC Islamic affairs analyst
Harsh sectarian sentiment in the Muslim world was until recently largely confined to the radical fringe.
Sectarian violence in Iraq has inflamed feelings across the region
Conservative Sunni clerics in Saudi Arabia routinely denounced Shia as heretics.
Jihadists of the al-Qaeda type launched brutal attacks on Shia mosques and clerics in Iraq.
Now, under the pressure of events in Iraq, there are signs that sectarianism is going mainstream.
In January an editorial in al-Ahram, a newspaper widely seen as the voice of the Egyptian state, declared: "Iran is working actively towards spreading Shia doctrine even in countries which do not have a Shia minority... paving the way for reviving the dreams of the Safavids."
The Safavids were the 16th Century Persian dynasty which introduced Shia Islam to Iran - and ruled large parts of Iraq.
The charge that there is an Iranian-inspired campaign to convert Sunnis to Shia Islam is now commonplace in the Arab world.
But the evidence suggests the number of converts is tiny.
A Sunni Arab order
Two developments - the emergence of a Shia-dominated government in Iraq and the growth of Iranian influence in the region - are provoking a Sunni backlash.
"Whether we like it or not," says former Iraqi Defence Minister Ali Allawi, a prominent Shia politician, "the former Baath regime in Iraq did represent a kind of Sunni Arab order."
The collapse of the regime, he says, was bound to set off regional alarm bells.
The Arab world is run by a set of Sunni elites for whom Shia power is an unwelcome novelty.
Juan Cole, a Middle East expert at the University of Michigan, sees a number of factors at work.
"I think there is, first, a geopolitical struggle between Iran and the Sunni Arab regimes over who is the regional superpower. Is it Iran? Is it Egypt? Is it Saudi Arabia?"
A second factor is that "the have-nots of the Middle East are turning to Iranian-style Shia radicalism to express their discontent with the status quo".
A striking example is Hezbollah, the dominant force among Lebanese Shias.
Hezbollah's conflict with Israel has propelled it to regional prominence
The movement's success in last summer's conflict against Israel propelled it to regional prominence.
The feeling of being "have-nots" dates back to the origins of Shia Islam in a dispute in the 7th Century AD over who should succeed the Prophet Muhammad.
The Shias lost out, and ever since have felt themselves to be a disadvantaged minority. Now the Shias are seen to be on the march.
The notion of an Iranian-dominated "Shia crescent" stretching from Lebanon to Afghanistan may be exaggerated, but it's proving hard to shake off.
The sectarian card
The Sudanese writer Abdelwahab el-Affendi, a well-known Sunni Islamist, thinks Arab perceptions of Iran are changing.
At the time of the Iranian revolution in 1979, he says, Sunni and Shia Muslims alike were attracted by its pan-Islamic activism and support for the mustazafin, the dispossessed.
Now, he argues, Iran has a less progressive and more narrowly Shia image.
"They have played the sectarian card [by their support for Shias] in Afghanistan. They have played the sectarian card in Iraq. I think they have been short-sighted."
But if regional states are fuelling sectarianism, Ali Allawi thinks the Bush administration's policy of piling up pressure on Iran may make matters worse.
"The way things are unfolding now it seems the Americans may even be encouraging the sectarian trend - not in Iraq but in the nearby countries."
Washington's attempt to bring together a group of Sunni Arab states - including Saudi Arabia and Egypt - into an anti-Iranian alliance could backfire.
Many in the region are fearful that the sectarian genie is out of the bottle, and that it will be hard to put it back.