Afghanistan was supposed to be the good war. Unlike Iraq, foreign troops had gone in under a United Nations mandate and the majority of people had welcomed them as harbingers of peace after decades of Afghan fighting Afghan. When the Royal Marines drove into Kabul in December 2001, as the first foreign peacekeeping forces following the ousting of the Taliban, locals came out into the streets to cheer.

More than six years on, that goodwill has well and truly evaporated. Despite a tenfold increase to 47,000 in peacekeeping forces (or international stabilisation assistance as it is known in UN jargon), the Taliban are on the rise and much of the country has become a more violent place. Last year was Afghanistan's bloodiest since 2001; 8,000 people were killed in fighting, among them 1,500 civilians, according to the United Nations. Not only have the Taliban been steadily creeping beyond their traditional areas of influence in the south and the east of the country, but 2008 has seen them launch audacious attacks in the Afghan capital.

Last week, the out-going Nato commander described the war as “under-resourced”. Yet far from inspiring nations who have troops there to send reinforcements, this is more likely to raise the Iraq-like question of whether they are achieving anything by their presence. It is a debate that has been raging in Canada - which has taken heavy casualties - and it would be surprising if it does not soon become an issue in Britain and America, the two nations that contribute the most troops and are engaged in the bulk of the fighting.

The past few years have seen a slew of books on what went wrong in Iraq - now it's Afghanistan's turn. James Fergusson's A Million Bullets focuses on the experience of the British troops sent to Helmand in southern Afghanistan in April 2006. What had been billed as a reconstruction mission, where it was famously hoped not a single shot would be fired, has ended up as the army's fiercest fighting for 50 years.

Fergusson has been slightly pre-empted by Patrick Bishop's excellent 3 Para, which tells in gripping detail the story of the first combat troops on the ground, in their own words. But where Bishop's book is written in Boy's Own fashion, with little analysis of the situation, A Million Bullets is more introspective. Fergusson's aim is to answer the important question of what Britain is doing in Afghanistan and what, if anything, is being achieved. He also desires to show the war from the other side, and bravely (or foolishly, had it gone wrong) sets up a meeting with a Taliban commander. The Taliban tell him they are baffled as to why, after three past wars of which the first two were disastrous, the British would again become involved in Afghanistan. “A clever man does not get bitten by a snake from the same hole twice,” says one. But we learn little that we don't already know from Fergusson's meeting. The people we don't hear from - as is too often the case - are the ordinary Afghans.

Instead, Fergusson keeps getting drawn back to the action in Helmand in 2006. In an attempt to cover ground untouched by Bishop, he focuses on the non-Paras (the Gurkhas, the Fusiliers, the Royal Irish, the Household Cavalry and the RAF), all of whom feel they are the unsung heroes. With the usual squaddie resentment towards the Paras (who, they point out bitterly, received most of the medals for the tour), they are only too eager to tell their stories. Fergusson is clearly a sensitive interviewer and he elicits some fascinating material. But he is hampered by the fact that he was not there. Bishop was not there, either, yet in 3 Para he still managed to convey the heat, the dust and the fear of spending months under siege in platoon houses in small towns. Where Fergusson does succeed brilliantly is in detailing the emotional impact on soldiers - mostly in their twenties, who had never fired on anybody - killing for the first time and seeing comrades killed.

The most vivid part of the book is the story of the Fusiliers who were sent to the small market town of Nawzad (spelt Now Zad here) for two days and ended up staying 107, enduring 149 contacts. Not only did they feel abandoned to their fate, but lack of coordination between the British and the Americans left them having to deal with the fallout from attacks on locals by American special forces. One captain, with a 22-year-career in the army, tells how, before being deployed to Helmand, he had fired precisely 11 shots in anger, all in Northern Ireland. In just one hour in Nawzad (his first) he fired far more. Dean Fisher of the Fire Support Group fired 40,000 rounds. He described their five months under siege as worse than jail. “I've been in prison and at least you get electricity. And nobody's shooting at you. There were times when you'd just be lying in your trench and thinking to yourself, ‘What am I doing here?'”

Afterwards all of them had problems in readjusting, unable to explain what they had endured to friends and relatives. Inevitably, this put a strain on marriages - in 2006, one battalion alone experienced six divorces and 10 separations. One can only begin to imagine the agony of Fusilier Matt Seal, under siege in Nawzad, rushing to the long-awaited postbag hoping for a letter from home. Instead he found an envelope containing divorce papers.

Yet in some senses he is one of the lucky ones; 97 British servicemen have not made it home alive. Seal, like many others, must be asking is it worth it.

A Million Bullets by James Fergusson
Bantam Press £16.99 pp358 Buy the book £15.29 plus free delivery

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