The Samarra skyline is dominated by two sights, writes Ewen MacAskill.
The main attraction for most western tourists - and there was a trickle of these during Saddam Hussein's rule, though none now - was the spiral minaret, perhaps the most distinctive sight in the whole of Iraq. But the main attraction for Shia Muslims from the west, as for those visiting in greater numbers from Iran and elsewhere in Iraq, was always the golden mosque, destroyed today by Sunni gunmen.
The mosque is one of four Shia holy sites in Iraq, the others being Najaf, Kerbala and Baghdad. Pilgrims came to Samarra to pay homage to two imams buried there, Ali al-Hadi and al-Hasan al-Askari, the 10th and 11th Shia Imams; the 12th Imam, Mohammed al-Mahdi, disappeared there.
When news of the destruction reached Baghdad, Tehran, London and Washington, there was immediate recognition that this was an act with potentially enormous consequences.
Diplomats at the Foreign Office, conscious of the impact that the attack will have on Shia Muslims, urged Jack Straw, the foreign secretary, to put out a speedy reaction. He described it as a "criminal and sacrilegious act".
The Shia have been on the receiving end of some of the worst atrocities since the fall of Saddam Hussein, most of them caused by car bombs. But the Shia, who are the majority in Iraq, will see a direct attack on one of their holiest shrines as harder to forgive, taking them a step closer to civil war.
The majority of Sunni Muslims will also be appalled by the attack, both because it is a direct attack on a holy site, even if not one of theirs, and because they fear it will bring retaliation. The Sunnis more than any other group fear that they will lose out if the long-predicted civil war takes place.
The Sunnis are a minority in Iraq, but they were dominant under Saddam, and since the US-led invasion in 2003 the insurgency has been Sunni-led.
The nationalist part of the insurgency targets US troops and Iraqi police and government officials. But a group led by Abdul Musab al-Zarqawi, a Sunni who has links with al-Qaida, has been targeting the Shia, trying to spark a civil war in the hope of benefiting from the resulting chaos.
The Shia, in turn, have been victimising the Sunnis. In control of the government, they have used the levers of power - the police and the ministry of interior - to settle scores from the Saddam era and to imprison and assassinate Sunni leaders.
The US and British have been trying for weeks to persuade the Shia parties, victors in the December elections, to form a coalition government that includes Sunnis. They argue that such a move could split the insurgency and prevent a civil war.
In spite of the widespread protests throughout Iraq, Shia religious and political leaders called for restraint.
But the issue is not one for Iraq alone: the golden mosque is a site of pilgrimage for the Shia of Iran and is of special significance for the Iranian president, Mahmoud Ahmadinejad, a follower of the 12th imam. The attack could provoke greater Iranian involvement in Iraq.
