By Jonathan Beale
BBC News, Washington
Haditha will add to the sense of despair and gloom coming out of Iraq that now surrounds the White House.
Women and children died in the attack in Haditha
Even President George W Bush has recently conceded that America is "not winning" the war.
Haditha will make it all the harder to "win" - not just the real battles against insurgents and preventing a civil war.
Haditha makes it harder to win the hearts and minds of ordinary Iraqis and an increasingly sceptical American public.
The US military had been bracing itself for bad news. In the wake of the Haditha allegations it announced that troops would undergo a "Core Warrior Values" course - essentially ethics training.
At the time Lt Gen Peter Chiarelli - commander of US led forces in Iraq - said: "It is important that we take time to reflect on the values that separate us from our enemies".
Pattern of abuse
The army and marines already knew that the slaughter of innocent Iraqi civilians would make their job more difficult. They hoped that this would be seen as an isolated incident.
But there is a growing list of American soldiers and marines accused of breaking the law - and what is more any moral code.
First there was Abu Ghraib. The abuse of prisoners has caused lasting damage to America's image around the world. More recently there is Hamdaniya and Mahmudiya.
In short there is enough for America's enemies to point to a pattern of injustice and probably enough to turn more Iraqis against the occupiers.
President Bush hopes that by investigating and prosecuting any perpetrators that America will still be seen as righteous. In truth the damage has already been done.
But whatever happened at Haditha is unlikely to have the same impact as the events in Abu Ghraib.
There is the confusion surrounding the "fog of war". Yes there is damning eyewitness accounts. But unlike Abu Ghraib there are no pictures to tell us exactly what happened.
Nor is this the first time that Americans have heard accounts of atrocities on their own side. The shock factor is not as great as it was in Abu Ghraib - even if marines are found guilty.
Some have suggested that Haditha might go down in history as another "My Lai Massacre". The murder of hundreds of Vietnamese civilians in March 1968 by US soldiers turned many Americans against the Vietnam war.
But there is no sense that the American public view Haditha on the same scale.
Nor is their any evidence that this event will turn Americans against their military. There is still broad support for US troops - if not for the war they are fighting.
But Haditha is likely to have an impact on the morale both of the US public and US troops.
Allegations like this inevitably erode the President's moral certainties about the Iraq war. It also raises serious question for US forces.
For the marines it will damage the "esprit de corps". Was there a deliberate cover up? More widely, it may raise questions about the rules of engagement. Will a soldier fear prosecution if he returns fire and accidentally kills a civilian?
In short Haditha - whatever happened - will go down as another reason why America is losing faith in this war.