Exhaustion and combat stress are besieging US troops in Iraq as they battle with a new type of warfare. Some even rely on Red Bull to get through the day. As desertions and absences increase, the military is struggling to cope with the crisis

Lieutenant Clay Hanna looks sick and white. Like his colleagues he does not seem to sleep. Hanna says he catches up by napping on a cot between operations in the command centre, amid the noise of radio. He is up at 6am and tries to go to sleep by 2am or 3am. But there are operations to go on, planning to be done and after-action reports that need to be written. And war interposes its own deadly agenda that requires his attention and wakes him up.

When he emerges from his naps there is something old and paper-thin about his skin, something sketchy about his movements as the days go by.

The Americans he commands, like the other men at Sullivan - a combat outpost in Zafraniya, south east Baghdad - hit their cots when they get in from operations. But even when they wake up there is something tired and groggy about them. They are on duty for five days at a time and off for two days. When they get back to the forward operating base, they do their laundry and sleep and count the days until they will get home. It is an exhaustion that accumulates over the patrols and the rotations, over the multiple deployments, until it all joins up, wiping out any memory of leave or time at home. Until life is nothing but Iraq.

Hanna and his men are not alone in being tired most of the time. A whole army is exhausted and worn out. You see the young soldiers washed up like driftwood at Baghdad's international airport, waiting to go on leave or returning to their units, sleeping on their body armour on floors and in the dust.

Where once the war in Iraq was defined in conversations with these men by untenable ideas - bringing democracy or defeating al-Qaeda - these days the war in Iraq is defined by different ways of expressing the idea of being weary. It is a theme that is endlessly reiterated as you travel around Iraq. 'The army is worn out. We are just keeping people in theatre who are exhausted,' says a soldier working for the US army public affairs office who is supposed to be telling me how well things have been going since the 'surge' in Baghdad began.

They are not supposed to talk like this. We are driving and another of the public affairs team adds bitterly: 'We should just be allowed to tell the media what is happening here. Let them know that people are worn out. So that their families know back home. But it's like we've become no more than numbers now.'

The first soldier starts in again. 'My husband was injured here. He hit an improvised explosive device. He already had a spinal injury. The blast shook out the plates. He's home now and has serious issues adapting. But I'm not allowed to go back home to see him. If I wanted to see him I'd have to take leave time (two weeks). And the army counts it.'

A week later, in the northern city of Mosul, an officer talks privately. 'We're plodding through this,' he says after another patrol and another ambush in the city centre. 'I don't know how much more plodding we've got left in us.'

When the soldiers talk like this there is resignation. There is a corrosive anger, too, that bubbles out, like the words pouring unbidden from a chaplain's assistant who has come to bless a patrol. 'Why don't you tell the truth? Why don't you journalists write that this army is exhausted?'

It is a weariness that has created its own culture of superstition. There are vehicle commanders who will not let the infantrymen in the back fall asleep on long operations - not because they want the men alert, but because, they say, bad things happen when people fall asleep. So the soldiers drink multiple cans of Rip It and Red Bull to stay alert and wired.

But the exhaustion of the US army emerges most powerfully in the details of these soldiers' frayed and worn-out lives. Everywhere you go you hear the same complaints: soldiers talk about divorces, or problems with the girlfriends that they don't see, or about the children who have been born and who are growing up largely without them.

'I counted it the other day,' says a major whose partner is also a soldier. 'We have been married for five years. We added up the days. Because of Iraq and Afghanistan we have been together for just seven months. Seven months ... We are in a bad place. I don't know whether this marriage can survive it.'

The anecdotal evidence on the ground confirms what others - prominent among them General Colin Powell, the former US Secretary of State - have been insisting for months now: that the US army is 'about broken'. Only a third of the regular army's brigades now qualify as combat-ready. Officers educated at the elite West Point academy are leaving at a rate not seen in 30 years, with the consequence that the US army has a shortfall of 3,000 commissioned officers - and the problem is expected to worsen.

And it is not only the soldiers that are worn out. The wars in Iraq and Afghanistan have led to the destruction, or wearing out, of 40 per cent of the US army's equipment, totalling at a recent count $212bn (£105bn).

But it is in the soldiers themselves - and in the ordinary stories they tell - that the exhaustion of the US military is most obvious, coming amid warnings that soldiers serving multiple Iraq deployments, now amounting to several years, are 50 per cent more likely than those with one tour to suffer from acute combat stress.

The army's exhaustion is reflected in problems such as the rate of desertion and unauthorised absences - a problem, it was revealed earlier this year, that had increased threefold on the period before the war in Afghanistan and had resulted in thousands of negative discharges.

'They are scraping to get people to go back and people are worn out,' said Thomas Grieger, a senior US navy psychiatrist, told the International Herald Tribune in April.

'Modern war is exhausting,' says Major Stacie Caswell, an occupational therapist with a combat stress unit attached to the military hospital in Mosul. Her unit runs long group sessions to help soldiers with emerging mental health and discipline problems: often they have seen friends killed and injured, or are having problems stemming from issues at home - responsible for 50 to 60 per cent of their cases. One of the most common problems in Iraq is sleep disorders.

'This is a different kind of war,' says Caswell. 'In World War II it was clear who the good guys and the bad guys were. You knew what you would go through on the battlefield.' Now she says the threat is all around. And soldiering has changed. 'Now we have so many things to do...'

'And the soldier in Vietnam,' interjects Sergeant John Valentine from the same unit, 'did not get to see the coverage from home that these soldiers do. We see what is going on at home on the political scene. They think the war is going to end. Then we have the frustration and confusion. That is fatiguing. Mentally tiring.'

'Not only that,' says Caswell, 'but because of the nature of what we do now, the number of tasks in comparison with previous generations - even as you are finishing your 15 months here you are immediately planning and training for your next tour.' Valentine adds: 'There is no decompression.'

The consequence is a deep-seated problem of retention and recruitment that in turn, says Caswell, has led the US army to reduce its standards for joining the military, particularly over the issue of no longer looking too hard at any previous history of mental illness. 'It is a question of honesty, and we are not investigating too deeply or we are issuing waivers. The consequence is that we are seeing people who do not have the same coping skills when they get here, and this can be difficult.

'We are also seeing older soldiers coming in - up to 41 years old - and that is causing its own problems. They have difficulty dealing with the physical impact of the war and also interacting with the younger men.'

Valentine says: 'We are not only watering down the quality of the soldiers but the leadership too. The good leaders get out. I've seen it. And right now we are on the down slope.'

'War tsar' calls for return of the draft to take the strain

America's 'war tsar' has called for the nation's political leaders to consider bringing back the draft to help a military exhausted by wars in Iraq and Afghanistan.

In a radio interview, Lieutenant General Douglas Lute said the option had always been open to boost America's all-volunteer army by drafting in young men in the same way as happened in Vietnam. 'I think it makes sense to consider it,' he said. Lute was appointed 'war tsar' earlier this year after President Bush decided a single figure was needed to oversee the nation's military efforts abroad.

Rumours of a return to the draft have long circulated in military circles as the pressure from fighting two large conflicts at the same time builds on America's forces. However, politically it would be extremely difficult to achieve, especially for any leader hoping to be elected in 2008. Bush has previously ruled out the suggestion as unnecessary.

Lute, however, said the war was causing stress to military families and, as a result, was having an impact on levels of re-enlistment. 'This kind of stress plays out across dinner tables and in living-room conversations within these families. Ultimately the health of the all-volunteer force is going to rest on those sorts of personal family decisions,' he said.

A draft would revive bad memories of the turmoil of the 1960s and early 1970s when tens of thousands of young men were drafted to fight and die in Vietnam. Few other policies proved as divisive in America and the memories of anti-war protesters burning their draft cards and fleeing to Canada are still vivid in the memory.