Leading article: Alarming hints of future power struggles in Iraq

The eruption of widespread violence in Baghdad within hours of the withdrawal of American troops from Iraq is ominous. Children at school and commuters on their way to work were targeted in what looks like a deliberate attempt to foment sectarian tensions. President Obama's claim that US forces were leaving behind "a sovereign, stable and self-reliant Iraq" rings very hollow.

The deal Washington did between the Shia, Sunni and Kurdish sections of the Iraqi population was always uneasy. The danger of its fragmenting, now that the nine-year US occupation is over, is very real. It is unclear who was behind the explosions, but Sunni and Shia have each been quick to blame the other. Either way, it is clear that there are strong forces in the country who have been waiting for this moment to make their move to achieve supremacy.

What will happen now? One possibility is that the Shia Prime Minister, Nouri al-Maliki, will move to replicate Saddam Hussein as the strongman of Iraq. He is already deploying the state security apparatus against his Sunni critics on the grounds that they have sympathies with Saddam's banned Baath Party.

Mr Maliki has issued an arrest warrant for the country's top Sunni politician, Vice-President Tariq al-Hashemi, on what appear to be trumped-up terror charges. And he has asked parliament to remove Iraq's other most senior Sunni politician, Saleh al-Mutlaq, from office as Deputy Prime Minister. Sunni conspiracists are already announcing that there has been a secret deal between Mr Maliki and Washington that he should become Iraq's new dictator.

Mr Hashemi has fled to Iraqi Kurdistan to ask for the protection of the regional Kurdish government. The Kurds, who are the third major ethno-religious grouping in Iraq, have previously been the only ones able to play honest broker between Sunni and Shia, though neither of those factions backed the idea of a federated Iraq with the Kurds as an autonomous region. But the balance has shifted. Now Sunni-majority provinces which rejected the federal principle have begun to embrace the notion, to the fury of the Shias. The power-sharing deal devised by the Americans, hammered out in nine months of tortured post-election wrangling, is falling apart.

The biggest single winner in all this is neighbouring Iran. The overthrow of Saddam brought an end to the dominance of Iraq by a Sunni minority. Democracy has given voice to the Shia majority who feel a natural affinity to Shia Iran, creating a greater bulwark against US influence in the region. Iran is operating behind the scenes to bolster Mr Maliki with Iranian-backed militias, who are already a powerful force in Iraq. At a time when the West is talking of oil sanctions on Iran, and sections of the Israeli leadership are threatening a military assault on Iran's nuclear programme, Tehran must be pleased at developments in Baghdad.

Further complications arise from the turmoil in Syria, whose Alawite Shia leaders rule over a largely Sunni nation. If the besieged regime of President Bashar al-Assad were to collapse in Damascus, the Sunni-Shia faultline would shift tectonically. Into all that may be added the influence of Saddam's Ba'athist remnant and that of militant Salafist jihadis. These two powered the Sunni-based insurgency which killed thousands of Shia during the last outbreak of sectarian violence five years ago. They are still around. Indeed, many suspect that the degree of co-ordination involved in this week's bombings suggests that al-Qa'ida, whose adherents are mainly Sunni, may be involved.

It is a potent cocktail. What happened in Yugoslavia after the strongman Marshal Tito died could be about to be repeated in the explosive Balkanisation of Iraq. There can be no saying what will rush into the vacuum the US has left.