No one knows what triggered a US army staff sergeant's calm progress on 11 March through two tiny villages in the Panjwai District of Kandahar Province, murdering, we are told, as he went. Clearly something switched off his moral reflexes. In due course, as the case makes its slow progress through the US court martial system, we will discover more. What we do know is that he is a family man and that in addition to his time in Afghanistan he had served three tours in Iraq.

The US and the UK - or rather, our armed forces - have been at war now for more than ten years. Because of the way British forces are organised, the campaigns have been fought as a series of six-month "tours" of duty for those involved. Twice a year, the entire force comes home and is replaced by another.

The moment you arrive at your post in Afghanistan (or "in theatre") you know that you have 186 days to go. For those in an infantry role, patrolling the ground, these days are very often savage and stress-filled. I have been told that before some patrols in areas particularly infested with improvised explosive devices, soldiers will throw up with fear.

Many of our soldiers - those in 16 Air Assault Brigade, for example - have done as many as three tours in Helmand. Some, especially in "higher-end" roles such as special forces, have done many more, in addition to the tours they may have spent in the Iraq campaign. All this is against a background of a dubious strategy and shifting aims.

Yet many US soldiers consider the British routine to be somewhat soft. The US army sergeant accused of the Panjwai murders was part of a brigade serving a one-year tour. Until recently the usual duration of US deployments was 12 or even 15 months. This long tour is a legacy of Vietnam. The difference was that, in Vietnam, unless a soldier actively wanted to go back to serve in combat, he served only one tour. Even so, anyone with the slightest knowledge of what happened in south-east Asia will know what a single year's combat can do to a man.

Revolving door

Within the US armed forces it is now common for soldiers to have served four- or five-year-long combat tours over the past decade. This intensity of deployment to nerve-shreddingly dangerous areas is unprecedented and its effects are becoming, to put it mildly, apparent.

For those people there is, literally, no peace. There is a revolving door between daily risk of death or permanent disability, returning to an often unfamiliar and uncomprehending home, working up to the next deployment and return to the battlefield. Little attention is given to whether or not the soldier is fit for task. As one US veteran put it to me, "If the guy is standing and wants to go, he's going. It makes for less work for everyone."

Most soldiers are remarkably normal, life-affirming young men and women. Yet regular exposure to killing, dying and maiming will change everyone, regardless of training, experience or perceived toughness. The question here is how that change will manifest itself. Some will suffer post-traumatic stress disorder in silence. Some will turn on their families. There are those who become convinced advocates of more considered military intervention. Some may become pacifists; others will sign up as mercenaries. A tiny minority will turn on the nearest defenceless scapegoat, as US marines did at Haditha in Iraq in 2003 or US soldiers did in the "killing for sport" case last year. No one remains unaffected.

Britain's servicemen are no more virtuous than those of other armies, although they are more disciplined and professionally effective than most. These qualities, together with their esprit de corps, exert positive pressures against the kind of thing we have occasionally seen from US units. It helps that we tend not to recruit people into our services as an alternative to jail, as US forces have all too often done. Yet British soldiers, too, are capable of serious war crimes.

It is lethally dangerous for independent journalists or human rights monitors to venture into most of southern Afghanistan. Against a background of so little active scrutiny, rumours circulate of killings of prisoners or the covering up of civilian deaths by US and even British troops. Whether these prove to be exaggerations or simply untrue will eventually become clear.

As the years go on, researchers and journalists will gain access to areas now closed to them and we are likely to hear of more such atrocities by Nato troops.

We don't have any idea what, if anything, motivated the soldier in Panjwai to do what he is said to have done. He must take personal responsibility for his actions. The rest of us must reflect on our own willingness to send young men and women into wars such as this, time after time.

Frank Ledwidge is the author of "Losing Small Wars: British Military Failure in Iraq and Afghanistan" (Yale, £20)