Long ago and across the world in Vietnam, I had the job of persuading enemy soldiers to leave their government to join "our side" in the long struggle there against revolutionary socialism. Some of my experiences could be replicated in places such as Iraq and Afghanistan, although the recent news makes me wonder if it's still possible to bring people over to our side.

The names we hear daily in the news – "Haditha," and "Hamandiya" among several others – represent serious investigations into atrocities allegedly committed by American troops. It's impossible to say now what the outcome of these investigations will be. Many of the allegations involve the treatment of Iraqi and Afghan civilians. Some include people who were clearly combatants on the other side in the war.

The responsibility of our soldiers – or anyone's soldiers – to safeguard non-combatants is crystal clear in our law and in international law. The problem of how to deal with enemy fighters is another and more complicated issue.

At the commencement of this war on terror the Bush administration decided that enemy fighters would not be considered "prisoners of war," although they would be afforded comparable protections. This judgment, in my view, has made possible the questionable internment and interrogation facility at Guantánamo, "rendition" of prisoners to countries that are known to torture prisoners, such as Egypt, and a general lowering of standards in the treatment of prisoners in places such as the Abu Ghraib prison complex. From personal experience as a military intelligence officer who dealt with prisoners of war in Vietnam, I can tell you that the rules were quite different.

In Vietnam, enemy prisoners of war were treated in accordance with the Geneva Conventions and were given the POW designation. Many people have seen photographs of American or South Vietnamese soldiers with prisoners from the other side, the Viet Cong and North Vietnamese Army. Although there were undoubtedly instances in which individual Americans abused prisoners, I would defy anyone to provide photographic evidence of such abuse in a facility for the detention of enemy prisoners of war in Vietnam.

The enemies captured in Vietnam were held by US or South Vietnamese military police (MPs), interrogated by US Army or South Vietnamese military intelligence, and then sent to prisoner-of-war camps that were run by the South Vietnamese Army under the tutelage of American MP advisers.

Some exceptions applied. Underground political cadres (communist politicians secretly running a shadow government), for example, were not considered to be prisoners of war because they were neither soldiers nor organized guerillas and thus were not protected by the Fourth Geneva Convention on the treatment of prisoners of war. They were treated as criminals and traitors to the South Vietnamese state.

Enemy intelligence personnel apprehended in civilian clothes were subject to the sanction traditionally reserved for spies. There's the famous picture of South Vietnam's chief of police, for example, shooting a captured North Vietnamese intelligence officer in the street during the 1968 Tet offensive. The South Vietnamese general thought he was acting within his legal rights.

Nevertheless, the great majority of captured enemies, and by that I mean soldiers and guerrillas captured on the battlefield, went to prisoner-of-war camps.

These camps were located throughout the country. They housed thousands of enemy soldiers until the end of the war under conditions that met the standards of the International Committee of the Red Cross, the organization charged with monitoring adherence to the Geneva Conventions.

Part of my job that year was proselytizing in these camps, trolling for those who might want to change sides. I visited a number of these camps in 1972 and did not see anything very objectionable about them. When the war finally ended, these imprisoned soldiers were returned to their own side.

But as in any war, soldiers who are not so firmly anchored to one side can be persuaded to "come over." Often these men are among the most intelligent and experienced, who have come to see war itself as a cynical game played by the powerful at the soldiers' expense.

Hundreds of prisoners decided to change sides during the Vietnam War and join with US or South Vietnamese forces. One of the most useful projects that the "turncoats" served in were the "Kit Carson Scouts." These former enemy soldiers wore our uniforms, bore arms as part of our combat forces, and accompanied our own soldiers in the field. Their knowledge of the enemy's methods and habits proved invaluable. After demonstrating their loyalty to the American forces during the war, many of them came to live in the US.

I talked with a lot of prisoners that year. But one stands out. I was "scouting" in the POW camps for someone suitable for a special project that my unit was "running." We needed an enemy officer – an expert resource – to advise our intelligence analysts. We were notified that there was someone in a facility near Saigon who might be interested. I drove out there to see a North Vietnamese lieutenant.

We spent the day in a whitewashed room, smoking Gitanes (French cigarettes), drinking tea, and chatting. The lieutenant had been a rifle company commander in the 325th NVA Division. He spoke excellent French and had graduated from a good school in the North.

Like me, he had served a previous tour of duty in South Vietnam. He was an Olympic competitor in marksmanship and had been sent to the Olympic Games in Europe after his first combat tour. When he returned home, he was upgraded from a sergeant to an officer and sent back to South Vietnam with the 325th – an outfit akin to the 82nd Airborne Division in our forces. During our chat, we discovered that we had actually fought each other a couple of times in the Central Highlands of South Vietnam in days gone by. This was a kind of bond.

At the end of the day, he said that he wanted to get out of "this place," and that he had no one to go home to in the North. He asked if I thought he could live in California "afterward." He left the camp with me to work with our forces against his former comrades. I hope he made it to the Golden State.

None of this sounds like Iraq. We don't seem to really grasp the variety of our enemies in Iraq. We don't seem to know how to take advantage of that. I hope I am wrong.

• Patrick Lang is former head of human intelligence collection and Middle East intelligence at the Defense Intelligence Agency.