When soldiers kill unarmed civilians the fallout can last for decades. It is the message of the film Bloody Sunday, and the American military has been learning the lesson the hard way in Iraq.

At the US army’s staff college at Fort Leavenworth, in Kansas, Paul Greengrass’s 2002 docudrama on the killings in Northern Ireland has become essential viewing for officers being trained in counter-insurgency techniques.

They are hoping that new outrages can be prevented by studying the past.

Shot in jerky, handheld style by the director of United 93, Bloody Sunday chronicles the confusion and chaos that led panicky British soldiers to fire on a crowd of civil rights marchers in Londonderry in 1972, causing 14 deaths. The mindset of politicians and army officers is brutally dissected as well as the actions of the troops.

“It’s a very tough movie,” said a senior US army officer. “It highlights the critical importance of making sure that what you do at the highest levels is understood all the way down at the squad level.”

The massacre at Haditha and the prisoner abuse scandal at Abu Ghraib have driven that point home. Whatever the culpability at the top, it is the actions of the lower ranks that sparked the horror. Every soldier, no matter how powerful or powerless, can affect the course of the war.

One of Bloody Sunday’s greatest admirers is Lieutenant General David Petraeus, head of Fort Leavenworth, who recommends it to the officers under his command.

Petraeus, who led the 101st airborne division, the “Screaming Eagles”, into northern Iraq at the start of the war, has been drawing up the American army’s new counter-insurgency doctrine, its first for 20 years.

The model of a modern general, the Ivy League-educated Petraeus was chosen for the task because of his success in winning the confidence of Iraqis in his area. He went on to train the Iraqi security forces before returning to America.

The manual will be presented for approval this month to General Peter Schoomaker, the US army chief of staff, before it is published later this summer.

“One of the paradoxes of counter-insurgency is that there are times when less force is more productive than more force,” a senior army officer said.

“The idea is that you end the day with fewer enemies than you started. If a tactical action nabs one really bad guy but creates 25 more bad guys because of the way it is conducted, it is not useful.”

Phrases that would have once been dismissed as politically correct by soldiers, such as fostering “cultural awareness” and creating “stakeholders” in society — among the Sunnis, for example — are in vogue. Petraeus has been telling

his men not to “cordon and search” houses in trouble spots but to “cordon and knock”, ie, ask permission to enter. It is advice that could have prevented the horror at Haditha.

The efforts of the British to recover from the experience of Bloody Sunday are also being studied. “Over time, some very thoughtful and impressive approaches were adopted,” one army lecturer said, citing the gradual reform of the army, police and judiciary.

It is not an entirely useful model. “Historically, insurgencies have taken a long time,” the lecturer reflected. “How long have the British been dealing with Northern Ireland?”

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