Iraq has become a test case for the American experiment in untrammeled military power, and it is proving a difficult one. Playwright George Bernard Shaw once observed that any political arrangement that depends on soldiers is not likely to continue long. With the excitement of the armored race to Baghdad now a distant memory, the Bush Administration finds itself face to face, perhaps even more than its predecessors in Vietnam, with what could be called the essential meagerness of the military instrument. It can be a key that opens the door for other kinds of action, but it cannot substitute for them.

The truth in Iraq has, from the start, been that the American "occupation", like most occupations, has never meant any kind of close military control of Iraqi society. Even if close control was desirable, American and other coalition troops are not present in sufficient numbers - nor do they have the language and other skills that would enable them to exercise it.

While those who predicted an unalloyed welcome for the Americans proved to be wrong, they were right to the extent that the U.S. occupation relies on the consent of important forces in Iraqi society and on the promise of beneficial political and economic changes. It is this consent that is now wavering as fighting spreads - and along with it the idea that the Americans are losing their way and have no clear idea how to reassert themselves.

The U.S. position in Iraq has rested until now not principally on military strength, but on the cooperation of two critically important Iraqi forces: the Shiite religious leadership in the south and the reconstituted Iraqi police, and to a lesser extent the army, in the center of the country. The political and military developments of the past few weeks have weakened both of these pillars. The very fact that American and other coalition troops are now involved in military action in and around some areas under the control of Sunni and Shiite insurgents, even if that control is unlikely to last, is an index of how serious the circumstances in Iraq have become.

The Americans have reached this pass for a variety of reasons. Their main support in the Sunni areas, has for quite a long time, been hacked away by an insurgency that has targeted the Iraqi security forces and managed to reduce them to frightened bystanders in several key areas. The U.S. forces then compounded the problem, at least in Fallujah, by launching more aggressive operations - a change that may have been connected to the rotation in of new units with "new" thinking.

The main U.S. support in the south, by contrast, has been damaged by the Americans themselves. When the Coalition Provisional Authority helped push through an interim constitution that was not to the liking of the senior Shiite clergy, they weakened the limited confidence the latter had that their purposes coincided sufficiently with those of the U.S. to form a basis for cooperation.

The worst recent American misstep came with the decision to take on Moqtada al-Sadr, the young, extremist Shiite leader who has built a considerable following among poorer Shiites in Baghdad and in the main Shiite cities. He is not a major religious figure, despite hailing from one of Iraq's most important clerical families, but he embodies the anti-American and anti-foreign mood of many ordinary Shiites. That mood has been kept in check only by the authority of Ayatollah Ali al-Sistani and other senior clerics.

But that authority rests, as one journalist with deep experience in Iraq puts it, on a popular consensus. In other words, the senior clergy may appear to lead but - to a considerable extent - they also must follow, which explains Sistani's reluctance to unreservedly condemn Sadr or to endorse the action of American troops against him and his militiamen. To align themselves completely with the Americans in a situation where Shiite civilians are dying is not a politically possible course for the senior clergy.

The mistakes the Americans have made in Iraq have been enumerated often. It was a mistake to disband the Iraqi army and to ban most Baathists, because that sent a signal to many Sunnis that they were to be excluded from any political dispensation, deprived the Americans of a security instrument they then belatedly had to reconstitute, and fed an oppositional mood.

It was a mistake to let ideological obsessions about the free market and lack of regulation govern economic policy. It was, and is, a mistake to let troops be governed so much by the idea of self-protection, although that is one of the lessons about the limits of military power.

But the biggest error was not to grasp how damaged Iraqi society had been by years of dictatorship, by sanctions and by the corruption, apathy and cynicism that grew behind the facade of Saddam's supposedly strong state.

This larger error was perhaps understandable, because Saddam's Iraq was not an easy society to read. But it meant that the Americans had less to work with than they had expected, which made it even more important to capitalise on the Iraqi security tradition, repugnant though in some ways that tradition is, and on the coincidence of interest with the Shiite leadership.

Iraq is not yet the defeat for the United States that it could become. But America is chastened and perplexed. The Bush Administration that believed so devoutly that it could move mountains may now know better. It may even grasp that the concept to which it has always paid lip-service - that it is Iraqis who will decide their own future - is now more than just useful rhetoric. It is Iraqis, in the accumulation of their choices, decisions and actions, who will largely decide whether America's intervention ends up as a success or as a failure.

The Americans went to Iraq to rescue the Iraqis, and now stand in need of being rescued themselves.

Martin Woollacott writes on international affairs for The Guardian, London, where a version of this essay first appeared.