In Helmand, they protect opium shipments, extort money from poppy growers and operate heroin labs. In Kunar, they smuggle timber and guns. In the Swat valley, they control emerald mines, selling gemstones on the black market. On both sides of the Afghan/Pakistani border, they run a brisk kidnapping racket, snaring wealthy local businessmen, diplomats and journalists from around the globe.
When people in the west imagine the Taliban, most think of bearded fanatics, battling from caves under the flag of radical Islam. Having studied their day-to-day activities for more than five years, when I think of the Taliban I think of Tony Soprano and his gang.
I am not suggesting that Mullah Mohammed Omar has developed a taste for Chianti, or opened a branch of the Bada Bing at his hideout in Pakistan. As a fighting force, the Taliban remain as determined as ever to drive western forces from Afghanistan, as proven by the rising Nato casualty figures. But examine how the Taliban fund themselves, and how they interact with the local community, and they start to look more like mafiosi than mujahedin.
It is hard to make sweeping generalisations about the post-2001 Taliban. There are three distinct factions of the movement on the Afghan side of the border, and a far more fractious set of local and regional extremist groups in Pakistan. However, there are broad similarities in the way these various organisations are structured and how criminal proceeds filter up the chains of command. The manner in which they interact parallels the often tumultuous relations between Mafia crime families, like the New Jersey and New York clans portrayed in The Sopranos. Sometimes they collaborate; sometimes they battle against each other.
Whether fighting or conspiring, it is virtually always about making money. Western military officials believe that as little as 5 per cent of
the insurgents are "true believers" in their cause. Most of the fighters are in it just to make a quick buck.
I traced the Taliban's criminal earnings from the poppy fields of Helmand to the moneychangers in Dubai. Far from the stereotype of a ragtag militia of Islamic zealots, I found Taliban forces operating within an elaborate criminal economy that was astonishing in its size and complexity. Al-Qaeda was part of the picture, too - protecting drug shipments as they left Afghanistan and playing a coordinating role between the various local and regional extremist groups operating along the border.
With the help of local researchers, I conducted interviews with poppy farmers, truck drivers and heroin lab workers who described how Taliban fighters protected and taxed poppy farms, opium convoys and drug refineries. I heard again and again from western troops in Afghanistan how they found huge stashes of heroin and opium every time they captured a Taliban hideout. In one recent operation in Helmand Province, British, US and Afghan forces seized 92 tonnes of heroin, opium, hashish, poppy seeds and precursor chemicals. It was the second-largest drug seizure in world history. "Narcotics trafficking and the insurgency overlap to a degree that it is almost impossible to separate them," an American special forces officer in Kandahar told me.
Since 2001, Taliban commanders have deepened their involvement in the opium trade, but powerful trafficking cartels still control the drug industry, now worth billions of pounds annually. In 2008, I tracked down Haji Juma Khan, the region's opium kingpin, to one of his homes in Quetta, Pakistan. I drank orange soda with his colleagues, who spoke openly about the multi-tonne heroin shipments he sent towards the southern coast. (Just three months later, western counter-narcotics agents lured Khan to Indonesia, where he was arrested and extradited to New York.)
On other research trips, I toured the Pashtun slums of the seaport Karachi, where colourfully painted trucks from Afghanistan rolled into gated compounds on the coast. I watched men in tiny wooden rowing boats load bundles on to larger dhows bobbing in the harbour.
I interviewed a former money launderer in a Dubai coffee shop, who explained how he made dirty cash clean with a few quick swaps on the unregulated hawala money exchange.
Over time, and hundreds of interviews later with people who worked in or investigated this criminal economy, it became clear to me that characterising the Taliban as Islamic crusaders had caused Nato commanders to underestimate their enemy. For years, many western officials - especially in Washington - ignored the economic miracle funding the Taliban's resurgence.
It is important to recognise that from one of the world's most remote and mountainous regions, where there is no major highway network, nor freight train service, nor even widespread literacy (much less BlackBerrys and wifi), Taliban insurgents and the drug traffickers with whom they collaborate have accomplished an astonishing feat: successfully integrating an agricultural product into the global economy.
From importing precursor chemicals to getting farm loans to thousands of small farmers to providing security for the shipments as they move across borders, co-ordinating and managing Afghanistan's mammoth opium trade is an organisational feat of the very highest order. In less than eight years, they have come to dominate the global market share, supplying more than 90 per cent of the world's opium.
In recent years, as the Pakistani Taliban have rolled across that country's north-west, I have watched the Taliban expand into various new moneymaking schemes, from extortion to good old-fashioned bank robbery.
Fighting this widening insurgency is going to be an immense challenge for the Nato alliance and the international community - not least because the governments of Afghanistan and Pakistan are themselves so riddled with corruption. But a good start will be to define the insurgents as what they really are, and to protect local populations who are the victims of their criminal activity. These are not holy warriors fighting for Allah, but criminals after the almighty dollar.