For decades, it has been generally accepted that the My Lai massacre of as many as 400 Vietnamese civilians by U.S. Army troops on Mar. 16, 1968 was a violation of official policy directives on the treatment of civilians in South Vietnam.

That was the conclusion reached in the most definitive official account of why My Lai happened  the final report by Gen. William Peers, who investigated the question of responsibility for the massacre in late 1969 and early 1970 for the Department of the Army and the Army Chief of Staff.

Documentary evidence from U.S. army archives shows, however, that the Peers report misrepresented a key directive from the top commander in Vietnam, Gen. William C. Westmoreland, describing it as calling for a blanket policy of humane treatment of civilians in villages controlled by the Communist movement.

The directive in question, a copy of which has been obtained by IPS, makes it clear that the policy of humane treatment did not extend to civilians in areas which had been under long-term Communist rule, as was the case with My Lai. That revelation would have placed the responsibility for the orders on the My Lai operation squarely on Westmorelands shoulders.

The Peers report found that the troops who entered My Lai and three other hamlets of the village of Son My had been led to believe that everyone in the village should be killed. Testimony before the Peers inquiry also showed that the platoon leaders involved in the operation had been given that same message by two company commanders.

The report concluded that the Task Force commander responsible for the operation, Col. Frank Barker, had failed to "make clear any distinctions between combatants and noncombatants in their orders and instructions." The result, it stated, was that he had "conveyed an understanding that only the enemy remained" in My Lai.

The report asserted, however, that there was no higher command responsibility for what happened in My Lai. It concluded that the policy guidance from Gen. William Westmoreland, the commander of all U.S. forces in Vietnam, was "consistent in adhering to the humane standard of protecting the civilians within the combat zone."

The most important document cited by the Peers report in support of that conclusion was Directive 525-3 from the Military Assistance Command, Vietnam (MACV), called "Combat Operations: Minimizing Noncombatant Battle Casualties."

The Peers report said that one of the "significant points" of the directive, first issued on September 7, 1965 and reissued in slightly revised form on Oct. 14, 1966, was that "Specified strike zones should be configured to exclude populated areas."

"Specified strike zones" was the term that replaced the original term "free fire zones" created in 1965 to refer to zones where air strikes and artillery fire could be used freely with the approval of the province chief  approval which was routinely given to U.S. forces.

That description of a key point in the directive, which avoided direct quotation from the document, made it appear that the noncombatant population was to be protected from indiscriminate U.S. firepower in all Viet Cong hamlets. The report stated without any qualification that "specified strike zones" were "usually free of any known populace."

But the actual text of Directive 525-3, a copy of which was obtained from the U.S. Army Military History Institute in Carlisle Barracks, Pennsylvania, makes it clear that the intention of Westmoreland and the U.S. military command in regard to hamlets like My Lai was exactly the opposite.

The five-page directive was explicit about its concern with minimizing such casualties in contested areas, where the population had not been under long-term Communist influence. As the directive explained, "The use of unnecessary force leading to noncombatant battle casualties in areas temporarily controlled by the VC [Viet Cong] will embitter the population, drive them into the arms of the VC, and make the long-range pacification more difficult and more costly."

But the directive made it clear that this motivation for humane treatment of civilians did not apply to those who had been under long-term Communist rule. A key point in the directive said, "Specified strike zones should be configured to exclude populated areas, except those in accepted VC bases."

The term "accepted VC bases" referred to large parts of South Vietnam, including Son My village and most of Quang Ngai province, where the Viet Minh movement had mobilized the population to fight against the French and where the Communist movement had strong organizations throughout the Diem regime and in the early years of the U.S. war in Vietnam.

The directive thus made it clear the U.S. military command’s policy was to consider the civilian population in long-term Communist base areas as the enemy which could be subjected to the same treatment as Communist military personnel. The Peers report description  which avoided quoting directly from the document  effectively covered up the actual intention of the command’s policy toward noncombatants in places like My Lai.

Directive 525-3 is not the only piece of evidence pointing to a military command policy of treating noncombatants in Viet Cong base areas as subject to indiscriminate violence. In his own memoirs published in 1976, General Westmoreland himself wrote that, once the "free fire zones" were established, "anybody who remained had to be considered an enemy combatant," and operations in those areas "could be conducted without fear of civilian casualties."

Westmoreland was even more explicit in a visit to a unit of the 101st Airborne Division called the Tiger Force in Quang Ngai province in 1967. As recounted by members of the Tiger Force who were present, and reported by Pulitzer Prize-winning Toledo Blade journalists Michael Sallah and Mitch Weiss, Westmoreland told them, "[I]f there are people who are out there  and not in the camps  they’re pink as far as we’re concerned. They’re Communist sympathizers. They were not supposed to be there."

That message gave the Tiger Force officers the idea that they were authorized to kill anyone who chose to remain in Viet Cong base areas. Sallah and Weiss found that Tiger Force had carried out no fewer than 19 killing sprees against civilians in "specified strike zones." The unit commanders justified the wanton murder of civilians to Army investigators by explaining that the creation of a free fire zone gave U.S. troops the right to "kill anything that moved."

The Peers report recommended disciplinary action against 30 Army officers, including two generals and four colonels. But when it came to his treatment of Westmorelands policy directives, Gen. Peers had a strong incentive to absolve him of any responsibility for My Lai.

James K. Walsh, Jr., who was Special Counsel to the Peers investigation, recalled in an interview with IPS that Peers had hoped to become commander of the 8th Army in South Korea after his service in Vietnam.

That meant that he had to have the support of the Gen. Westmoreland, who had become the Army Chief of Staff in 1968 and thus was in a position to determine whether he would get the choice assignment he wanted.

Unfortunately for Peers, Westmoreland was replaced as Chief of Staff by Creighton B. Abrams in June 1972, and Abrams was openly hostile to the whole Peers investigation, according to Walsh. Peers never got the 8th Army command and chose early retirement.

Gareth Porter is an historian and national security policy analyst. The paperback edition of his latest book, Perils of Dominance: Imbalance of Power and the Road to War in Vietnam, was published in 2006.