Bobby was small for a 16-year-old, hunched and skinny. He left school without qualifications and took up with a local gang until he found their exploits too wild. He was mad about sport, so when an opportunity arose to do volunteer coaching at a school across town, he jumped at it.

The gang didn't approve. They called on his mother and told her if he didn't quit, they'd stab him. Both she and Bobby took the threat seriously. Nonetheless, he defied them. His team leader collected him from home every morning and delivered him back at night. You might think Bobby lived in Sarajevo or Northern Ireland at the height of the troubles. But he is a Glasgow boy and I met him last year.

When I listened to this youth, I wondered what motivated him to risk his life on a daily basis. Was it courage or a desperate need to make something of his life? If it were the latter, he might now succeed. His volunteering earned him a placement with a leading sports club. I won't name it, just as I haven't used his real name.

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When I happened upon Bobby I thought he was untypical. Since then the news has carried stories of gang violence from teenage shootings in London to the death of 11-year-old Rhys Jones. Police are looking for a 15-year-old assassin. It is hard to believe someone so young could be so violent. We have seen it in African countries where children are kidnapped and forced into soldiering during civil wars. But to most people it is incomprehensible here, in a civilised welfare state, which may explain why we seem to be at such a loss to know how to deal with it.

How do we reclaim these violent youngsters? How do we prevent more following in their footsteps? A friend said that if I wanted to understand gang culture I should read James Patrick's book, A Glasgow Gang Observed. It took me two days and opened my eyes to the realisation that, within a city I thought I knew, there is a parallel universe. There is a world where teenagers call the shots and make the rules and enforce their will at knifepoint. There is a world where to be 16 or 17 with a weapon in your pocket is to be someone of importance. Children look up to you, men fear you and your peers give you respect.

You don't need looks or intelligence. Qualifications are a laugh and authority is something to be flouted. As careers go, you've already been through the children's panel, have an Asbo and are well on the road to prison. Every step the conventional world calls a descent into crime is viewed in this parallel one as a stripe on the sleeve. In gangland, you are a junior officer en route to becoming a general. If exams were a humiliation and job prospects begin and end with shelf-stacking, why wouldn't you join a gang?

James Patrick wrote his book in the late 1960s but so much of it seems to speak to today and our growing worries about gang violence. In his book there is a map of Glasgow's gang territories. A similar one exists today. The territories sit neatly within the poorest areas. Patrick is a pseudonym chosen by the author who was, when he researched the book, a teacher in an approved school. He befriended a pupil called Tim and, with the boy's connivance, accompanied him on his weekends out and joined the young man's gang. Tim was effortlessly well-behaved throughout the week at school. He was just above average intelligence and likeable. In deciding to open his teacher's eyes to gang life, he understood the risks far more clearly than the author did.

Initially, Patrick found himself accepted into a loose association of youngsters who spent their days hanging around street corners and their evenings in pubs and dance halls. Most of the time they told stories, or stared into space. Then, in a finger snap, violence could erupt. On his first evening, a man accidentally brushed against a gang member in the pub. He was bashed over the head with a glass bottle. He and his mate were then dragged to the floor where they were jumped on and kicked in the head. On this, as on all occasions, all gang members joined in. I was reminded of a pride of lions or pack of wild dogs, springing into action at the prospect of a threat or a kill.

The boys, for that's all they were, would send out challenges to enemy gangs and, mob-handed, do battle, fuelled by fear and aggression. Once the fight started they expected no mercy and gave none. Tim, so temperate throughout the week, demonstrated his leadership skills with hammer, club or blade.

In a matter of weeks the situation had become too hot for Patrick to handle but, during his time with the gang, he gleaned valuable information. Tim, for example, inherited his role from older brothers who had gone on to greater violence and were thus feared and respected. His father encouraged his participation and his mother ignored it. The gang offered Tim, and his contemporaries, a life. Outside it they were poor, uneducated nobodies. In the gang they had station and respect. Approved school, borstal and prison were regarded as unfortunate side-effects.

I might have put A Glasgow Gang Observed back on the bookshelf in the history section - for some of the gangs mentioned date back to the 1880s. However, a series of articles in the Evening Times in 2006 convinced me little had changed. Reporter David Leask witnessed gang warfare and discovered the gangs made appointments to fight. Weapons included golf clubs, metal poles, razors and knives. And gang membership still restricts its members to their own territory. When Leask told an unemployed Ruchazie youth there were jobs at Glasgow Fort, the boy looked horrified. He said: "I would have to get a number 38 bus and that goes through Garthamlock. There is no way I'm going to risk my life like that."

However, to those growing up in these areas, the advantages of gang membership outweigh the disadvantages. In fact, for the youth who is growing up without a stable family or much education, arguably it is an intelligent decision. It offers protection, companionship, acceptance, status, a peer group and purpose. That is what our side of the parallel universe has to understand. That is what we have to counter.

The UK is now faced with increasing gang violence. Direct comparisons cannot necessarily be drawn between Glasgow and England. There are issues such as race and drugs that differ but there must be common factors and lessons to be learned. Last week there was positive news from Strathclyde Police. For seven weeks it ran Operation Phoenix which involved taking 1000 teenagers from across the city and getting them involved in sports. Throughout the initiative gang fights in Glasgow dropped by 73% on Friday and Saturday nights.

Is the opportunity to go dancing, kayaking, walking and play football all that it takes to stop teenagers fighting? It is not quite so simple but isn't it heart-breaking to think our lack of imagination and inability to put ourselves into the shoes of these teenagers have failed so many at such tragic cost?

The war on gangs is unlikely to be easily won but Operation Phoenix points to part of the way forward. A Glasgow Gang Observed provides the key to the rest. Its message is writ large: teenagers need opportunities, not Asbos. They need arenas, other than the gang, in which to form friendships and develop in groups. They need opportunities to demonstrate prowess that is unrelated to street violence.