By Lin Noueihed
BAGHDAD, Feb 24 (Reuters) - In the past three years, Iraq has teetered on the brink of civil war more than once. But in the past 48 hours, since Wednesday's bombing of a revered Shi'ite shrine, the threat has moved much closer to reality.
With Sunni Muslim militants targeting the Shi'ite majority, and incensed Shi'ite militias retaliating, the threat of a full-blown sectarian conflict is now very real.
At the same time relations between Shi'ite factions are increasingly fraught, with occasional clashes in recent months, raising the risk of a chaotic free-for-all which analysts say could be much worse than a relatively clear-cut civil war.
"This isn't shaping up to be just a civil war -- it's worse than that. It's a war of all against all," said Toby Dodge, an Iraq expert at Queen Mary University, London.
"What we have is a security vacuum that has given rise to various different forces fighting each other for control ... it's much more fractured than a civil war."
Armed rivalry between two Shi'ite militias -- the pro-Iranian Badr movement and the Mehdi Army, which is loyal to anti-American cleric Moqtada al-Sadr -- could intensify.
For now Sadr and Abdul Aziz al-Hakim, the head of the Supreme Council for the Islamic Revolution in Iraq, to which Badr belongs, are formally standing together after the attack, probably by Sunnis, on Samarra's Shi'ite Golden Mosque.
But Badr and the Mehdi Army have clashed in cities in the mainly Shi'ite south in the past and their varying visions of Iraq's future constantly threaten to blow up into open conflict.
Simultaneously, Shi'ite militias and Iraq's mixed Sunni-Shi'ite security forces are facing threats from a range of violent Sunni groups -- nationalists, Baathists loyal to Saddam Hussein and Islamist militants, some of them foreign.
On Friday, members of the Mehdi Army confronted suspected Sunni gunmen in street battles in the capital.
The risk that a breakdown in authority leads to full-scale war is greatest in Baghdad and surrounding towns where there are mixed Sunni, Shi'ite and Kurdish populations.
In those areas Sunnis and Shi'ites have already been driven from neighbourhoods or whole towns in a process recalling ethnic cleansing in the former Yugoslavia during the Balkan wars.
In Baghdad's mixed Doura district, Sunni-Shi'ite killings over the past year have redrawn boundaries to the extent that residents are now challenged on and marked by their sectarian affiliation -- something alien to Iraqis just two years ago.
URBAN NIGHTMARE
If full-scale war were to break out, it is Baghdad, Baquba, Kirkuk and Mosul -- cities with large, mixed Sunni and Shi'ite, Arab and Kurdish populations -- that could bear the brunt.
Baghdad, with its highly inter-mixed population, could end up resembling Beirut, focal point of the Lebanese civil war in the 1970s and 80s, or Sarajevo -- or something worse.
"If you look at Baghdad, it would be quite difficult to cleanse. If you tried to split it up, it would make Sarajevo look like a tea-party," said Dodge.
Much now hangs on how Iraq's armed forces perform.
Shi'ites dominate the army, thanks to their community's resurgence after Saddam's overthrow. But there are also Sunni Arabs and Kurds, both in command and in the ranks.
"Will the military maintain its form or will it implode?" said Martin Navias, a scholar at the Centre for Defence Studies at King's College, London.
"It could very well disintegrate and if it goes, we've got a very serious problem. The next few days are critical."
Crucial too is the role of the 160,000 U.S.-led foreign troops in Iraq. The U.S. military was patrolling Baghdad streets on Friday, keeping the peace under curfew, but also trying to maintain a low profile so as not to incite further violence.
Their presence is a double-edged sword, keeping the lid on wider conflict, but also exacerbating anti-U.S. violence.
Civil war could prompt Kurds to break away in their northern redoubt, already effectively autonomous and relatively insulated from the violence that has scourged the rest of Iraq since 2003.
Collapse of central authority might tempt Kurds to make a grab for the disputed city of Kirkuk, with its substantial oil fields, though any such move would make Kurds living in Baghdad and other cities vulnerable to Arab retaliation.
Any Kurdish lunge for secession might encourage Shi'ites to follow suit in the oil-rich south, leaving Sunnis with an arid swathe of central Iraq -- and everyone fighting over Baghdad.
Analysts are not convinced of the likelihood of a three-way division, but most agree that the outbreak of widespread fighting in Baghdad would make for a very bitter civil war.
(Additional reporting by Luke Baker in London)
