Take for example Lieutenant Clay Hanna. He 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 center, 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.
Hanna's pale and blotchy skin is not the only indication of his malaise brought about by fatigue. His energy levels and memory, among others, have dipped to all-time lows.
For Hanna and the men he commands, life is nothing but Iraq.
Accordingly, a whole army is exhausted and worn out. Young soldiers can be seen washed up like driftwood at Baghdad's international airport, waiting to go on leave or returning to their units, sleeping on their body armor on floors and in the dust.
Where once the war in Iraq was defined in conversations with these men by untenable ideas such as 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'.
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 center. '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.'
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, totaling at a recent count $212 billion.