It was a single shot - nothing spectacular - but that split-second act of Iraqi resistance might well be recorded as the point at which America turned from liberator to occupier.
Attacks are taking their toll on US soldiers
The soldier who died was on a foot patrol through the Baghdad University.
There was no sign of imminent danger, according to the politics and engineering students who saw what happened.
The soldier was almost certainly feeling relaxed and at ease as he sipped his soft-drink in the stifling heat.
Like all American troops on patrol here, he was sweating beneath his Kevlar flak-jacket and helmet.
They provided no protection whatsoever from the man who walked through the lunch-time crowd, put a pistol to the back of the soldier's skull, and pulled the trigger.
The killing was an audacious strike that forced the US military planners here to once more re-think their strategy across Iraq.
"Every time there's another attack, our bosses look at it and work out how to avoid the same thing happening again," said Lieutenant Brian Kendrick of the 1st Armoured Division.
"We're getting new orders all the time, but I'm not sure how you stop that kind of thing, unless we give up the foot patrols. But they are the best way of getting in touch with people, and gathering intel (intelligence)".
As the steady drum-beat of attacks strike the coalition forces each day, the options for the military planners narrow.
'Hard to fight back'
There are no more foot patrols through the Baghdad University now.
The relationship between US forces and Iraqis has suffered
Soldiers hardly ever leave their armoured Humvee vehicles, and every Iraqi civilian is treated as a potential attacker.
And for every death, there are at least a dozen other attacks that do not make the daily press bulletins.
In military terms, they are barely a pinprick on the rump of the American military, but they are taking their toll on the individual soldiers.
"You can't ever relax here," said one.
"There's no obvious danger, but we've learned to our cost that as soon as you let your guard down, the bad guys whack us out of nowhere. But with so many civilians around, it's hard to fight back."
But some American troops are.
Soldiers at a checkpoint recently believed they had spotted a sniper preparing to attack from the roof of a nearby building.
They fired at the position, and went to see what was there.
They found they had indeed killed someone - an 11-year-old boy.
It is a complex, messy and badly defined battlefield that is driving the Americans ever further from the very people they are supposed to be liberating, and sapping morale at the same time.
"I don't mind doing my duty. That's why I signed up," Sergeant Todd Lewis said.
"But the problem is I don't know how long I'm going to have to do it. I was married two years ago, and I've only seen my wife for six months in that time.
"We usually know how long we're going to be away, but the most our bosses are telling us now is 'We'll try to have you home before Christmas'. I don't think they really know what they're doing. I certainly don't," he said.
In and out?
And so, the question of an exit strategy has now become central to the issue of flagging troop morale.
It exists in broad theoretical terms - the plan is to set up political structures, draft a new constitution, hold elections and then pray that the result will be a Western friendly and oil-rich government in Baghdad.
But that is not the kind of clear "roadmap", to borrow a term, with defined timetables and obvious way-points along the route that the Iraqi people or coalition soldiers want to see.
"First they said we'd be in and out as quickly as possible," said Sergeant Lewis.
"Now they're saying that we'll be here for as long as it takes to establish freedom and democracy. The longer I'm here, the less sure I am that it will happen."