For a century and a half valiant knights have stepped forward to pull the sword from the stone. They have tried to tame the Afghan. They all fail.

The latest is John Reid, the defence secretary. On Thursday he stood with the ghosts of his forebears in the Commons to announce the dispatch of the latest thin red line of heroes to defend Britain from the opium-soaked mujaheddin screaming at the gates of empire. The jingoism of the debate was typified by Liam Fox, of the opposition, who declared with the bravura of a drawing-room general that it was unacceptable either “to fail to act or to act and fail”. Unacceptable in SW1 may seem less so in the sandstorms of Helmand.

Britain is being set up by the Americans in Afghanistan. Donald Rumsfeld, the US defence secretary, regards that country as past history, a forward base for terrorist-hunting and “interrogation”. Some 10,000 American troops are still combing the mountains, seeking Osama Bin Laden with bounties and a disregard for law or international boundaries. Anyone can be killed anywhere, anyone bombed. As for “nation-building”, Washington is not interested. It has installed Hamid Karzai in Kabul and protects him. That is enough. He must drag himself elegantly round the conference circuit, pleading for aid that never comes.

As for helping Karzai control Afghanistan beyond Kabul, the Americans are leaving that to mercenaries. This means the old warlords in the north, and in the south a less reliable ally for which Rumsfeld has only contempt, Nato. Since Nato moved into the Afghan theatre it has fully justified Rumsfeld’s scepticism. It can agree on nothing, bickering over force levels, areas of operation and rules of engagement.

The Dutch promised troops but cannot commit. Reid announced last week’s deployment because British soldiers on stand-by are fed up with waiting. A total of 5,700 are to be committed to Afghanistan for three years at a cost of £1 billion. This is approaching two-thirds the numbers committed to Iraq. These wars were supposedly won years ago.

Reid’s policy is based on two assertions. The first is that Britain’s defence interests require sending an army to Afghanistan to prevent a Taliban revival. The second is that achieving this objective is “absolutely interlinked to countering narcotics”. Neither objective is achievable, least of all with a total Nato force just 9,000 strong. I doubt if 90,000 could do the trick. The expedition has built-in failure. It is quite wrong to send an army on such a mission.

Britain’s lead contingent is 3,300 troops in Helmand province in the south. Their objective, says Reid, is “counter-insurgency” and “securing a framework” but not “counter-terrorism”. This means reacting “robustly” if attacked but not going in pursuit, which is the task of America’s Operation Enduring Freedom, operating at large under American command. The implication is that British troops will act only in self-defence. How that turns back the Taliban tide is a mystery. It is just offering target practice for mujaheddin.

None of the senior soldiers to whom I have talked, let alone intelligence sources or United Nations drug officials, can make sense of Reid’s mission. Last December its thrust was described as poppy eradication. On Thursday Reid dropped the term but spoke euphemistically of “economic development” and “extending political power under democratic control”.

The political economy of Helmand means one thing: opium. The poppy crop is what oil is to Kuwait. With Taliban activity increasing, American tactics have been to tolerate the drug barons and their allies the warlords as a buffer against infiltration. Officials in Kabul have distanced themselves from Britain’s obsession with destroying poppies, apparently rooted in Reid’s belief that narcotic demand at home can be stifled by restricting supply abroad. It is a policy of market regulation that the Americans have grimly failed to validate in South America.

The West’s invasion of Afghanistan liberated the opium trade from a temporary suspension under the Taliban. The proposal of the Senlis research council to imitate Turkey and India and buy the Afghan crop for medicinal use was rejected by the British and Americans. This is despite there being a world shortage of opium-based morphine.

In response to Europe’s uncontrolled heroin demand, Afghan poppy production has expanded from six to 28 of its 32 provinces. In 2004 output broke all records (50% more than in the Taliban’s best year). UN monitors predict an increase again this year. UN reports describe a sophisticated industry supplying 87% of the world market and able to adjust acreage to world prices.

Opium is virtually Afghanistan’s only export industry, underpinning half the economy and almost all its disposable wealth. Any attempt to substitute other crops would be hopeless. Substitution was described to a recent US congressional committee as an “utter, abject, total failure”. Reid is blowing a further £20m on it, does nobody audit foreign aid?

Times Recommends

• Snow brings out the inner Tory in all of us
• Hail, Britain’s Indiana Jones of the Amazon
• Hewitt and Hoon – two rabbits, one hat