The British troops encamped outside Basra resemble Davy Crockett's colleagues in the Alamo. Nobody will come to their rescue. Their position is hopeless. They cannot win. They cannot escape. Their boss, the defence secretary Des Browne, has emphasised their political entombment by reneging on Gordon Brown's pledge to reduce their numbers by a half this spring. The American general, David Petraeus, yesterday said the same of his troops. He wants 140,000 of them to remain at the end of the current surge, dashing hopes that their numbers might come down. The occupation of Iraq is now officially indefinite. Too many politicians have too much to lose by contemplating retreat.

Reports from Iraq indicate that the military operations of the past two weeks in Basra and Baghdad were treated by the Americans (or at least by the vice-president, Dick Cheney, who visited Baghdad last month) as a milestone in the occupation of Iraq. Coalition spokesmen were to declare themselves in the dark, indicating Iraq's ability to look after itself and thus vindicating the 2006 surge policy.

The military objective of the policy was clear. The "awakening" movement divided the Sunnis into good guys and bad guys and has largely worked, backing to the hilt any gunmen likely to hold al-Qaida influence in check. A policy of ghetto-isation in Baghdad has kept Sunnis and Shias apart and reduced the body count.

Phase two was aimed at the Shias. The prime minister, Nouri al-Maliki, was told to show his ability to end the emerging civil war within the Shia community. On the one side were the trained Mahdist irregulars of Moqtada al-Sadr, on the other a ramshackle alliance of an unreliable army, a corrupt police and various Badr militias loyal to the moderate Islamic Supreme Council of Iraq. If Maliki could not put an end to Sadr's chaotic insurrection, progress in Iraq would be inconceivable.

The assault on Sadr's forces was double-pronged, intended to end their presence in Basra since the British withdrawal, and to suppress their stronghold of Sadr City in Baghdad. Maliki showed considerable bravery in going to Basra two weeks ago to oversee the assault of his troops on the rebels. But the operation failed, leaving some 500 dead and only an agreed ceasefire.

British forces on the outskirts of Basra offered the Iraqis surveillance, some occasional shelling and rescue snatch squads, but critically they did not intervene sufficiently to ensure victory for Maliki. While this proved that Maliki's forces could not handle Basra unaided, the value of such proof is moot. Britain can hardly now re-assert authority over Basra when Maliki has failed. Yet if Britain remains aloof during such critical encounters, why be in Iraq at all?

The operation in Sadr City has been different. With American armour fully deployed, the sprawling settlement has been subjected to another Falluja, with tanks and bombers seeking to terrify the indigenous population into turning against Moqtada. Here sheer firepower could win the day. The Shia religious leadership of Ayatollah Ali al-Sistani is being consulted on a theocratic ceasefire, to stop a Shia civil war - but as of yesterday there was no sign of this.

As Petraeus told Congress yesterday, the surge has been a partial success in that crudest of measures: body count. But what next? Maliki has shown that he still cannot command authority in Iraq's two biggest cities without calling on foreign firepower. Sunni warlords have been armed, ghettos created and the Mahdists possibly silenced for the time being. But these are sticking-plaster jobs. They have done nothing to bring Iraq's communities together in some sort of political concord. It has rather realigned them for future conflict.

Maliki's survival may be crucial to Iraq's stability, and the surge important to that extent. But as so often with military operations there has been no political follow-through. Iraq has not found a leader remotely capable of ruling his country autonomously. While the Sunnis may rest content for the time being in their partitioned statelet, that does not apply to the Shias. Here Sadr will present a menace to Maliki as long as the foreign occupation is in place, and certainly given this week's proposal to make it indefinite. Moqtada can turn his gunmen on and off at will.

In Vietnam, American military mythology holds that if only the army had been more steadfast in opposing the Tet offensive, it could have won the war. It was "lack of will" at home that led to eventual defeat. Hence there should be no lack of will in Iraq, or America's friends will fall before an army of Iranian imams.

There is no way of sustaining a client who no longer exists except by virtue of being sustained. The past fortnight has shown conclusively that the Maliki government is wholly dependent on America. The surge was a military tactic, not a strategy. It was supposed, in that old cliche, to "supply politics with a breathing space". But hundreds have continued to die, and Iraq's politics remain rooted in the embattled culture of the green zone. The truth is that there will be no peace within the Shia regions, no peace between Sunnis and Shias, and no resolution of the issues dividing Arabs and Kurds until the occupation is over. The occupation freezes politics. All else is tinkering.

It is a truism that somewhere between the government, the Mahdists, the Badr brigades, the army, the police and the Sunni warlords an understanding will one day be reached. When that will be depends on the length of the occupation. Departure was meant to be after six months and has been postponed five years, while western strategists test to destruction their belief in military conquest as their preferred route to nation-building.

The surge sheds no light on this route. Iraq remains the most wretched country in Asia, its children dying youngest, its minorities most terrorised, its infrastructure most wrecked. Politics is in suspense, and the middle classes in exile or living in perpetual fear of death. The claim that America and Britain, who created this mess, can best serve it by continuing to hang around, bombing and shooting, is laughable.

Maliki may be the west's baby, and leaving him on the bare mountain may be harsh. But tough love is long overdue. The new Iraq, whatever that may be, has not seen an end to the beginning of its misery, let alone a beginning to the end.

simon.jenkins@theguardian.com