The Foreign Secretary, who had been in Baghdad barely a day earlier, said the bomb that pulverised Samarra’s golden dome yesterday was the work of a few terrorists and was condemned by most Iraqis.

That is, by now, a stock response. We have heard it every time that violence jerks the country backwards from the tattered remains of the US vision of its future. Yesterday it seemed as if Iraq had been blasted back hundreds of years.

Yet Straw has half a point. For all the quibbling about when exactly to award Iraq the label of “civil war”, it is fair to say that if you’re in one, you know it. The violence makes daily life impossible. Everyone is on one side or the other. Iraq is not yet there.

But where Straw’s remark is too blithe is that it would not take much to get there. Yesterday’s blast points the way. Sunni bombers, setting out to destroy one of the most sacred shrines for Shias, aimed to spark a sectarian war. If they do it again on other sites, it is easy to see how they would grab their prize.

If impassioned crowds will riot so easily against Danish cartoons, what would they do after the destruction of their holiest sites? To see the face of civil war, you have only to multiply the wreckage of al-Askariya shrine across the country. Yesterday there was a foretaste of that spectacle. Shia gunmen in Baghdad attacked at least 17 Sunni mosques and shot dead a Sunni cleric from a mosque in north Baghdad.

The Samarra attack represented a change in the choice of target by sectarian terrorists — from people to religious symbols. Both are hideously destructive. The killings by Shias and Sunnis have accelerated partition of the communities. Those who felt safe no longer do, and leave. Mixed neighbourhoods become one-sided in weeks.

But the assault on the shrine may prove even more inflammatory. The “before” and “after” pictures of the dome, commanding television screens across the region, are as stark a message as a party political broadcast. And some politicians were keen to seize the cause. One of the most senior Shia political leaders blamed the bomb, through tortuous logic, on the US Ambassador. Abdul-Aziz al-Hakim, head of the Supreme Council for the Islamic Revolution in Iraq, said that Zalmay Khalilzad had provoked the attack by criticising Shia-led security forces.

Al-Hakim, a constant aggressive voice from the Shia side of politics, has struck a triumphalist tone since the Shia majority elected a Shia government, ending decades of Sunni rule: don’t give the Sunnis an inch; drive them out; don’t trust them. But his contribution yesterday was particularly ugly.

That is the case for pessimism; in Iraq, always an easy one. There are a few, more hopeful signs.

The most important is the call for peace and restraint from Grand Ayatollah Ali al-Sistani, the towering figure among the Shia clerics. At times of potential turmoil, he has emerged to call for calm. It is because of his interventions that the south of Iraq, overwhelmingly Shia and desperate to assert that identity after decades of bloody repression, has not erupted. Maybe he can do the same again. Ali al-Bayati, from the Iraqi Embassy in London, said that the people of Samarra, mainly Sunni, had looked after the shrine for 1,000 years and were appalled, like Shias, at its destruction.

The title of John O’Hara’s 1930s novel Appointment in Samarra is taken from an old Arab proverb: a servant, buying food in a Baghdad market, sees the figure of Death, who seems to threaten him. He grabs his master’s horse and flees to Samarra. The figure of Death says: “I did not threaten him; it was a start of surprise to see him here in Baghdad, for I have an appointment with him tonight in Samarra.”

The moral is that you cannot outride your fate. For Iraq, at the moment, that would be too passive and bleak a conclusion, even after these past weeks.

Yet the clear hope of yesterday’s bombers was to set off a chain of religious assaults that join together to form the path to civil war.

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