If Britain has forgotten, at least Obama appears to be learning from America's equivalent example, Vietnam. The drift to a repeat of that catastrophe is the last thing his presidency needs just now. He can see that the occupation of Afghanistan has made every mistake in the invader's handbook. It has been Vietnam for slow learners.

There was the insertion of too many troops to make the invasion not an occupation, but too few to suppress the insurgency. There was the concept that aid could install democracy faster than occupation would create antibodies. There was the naivety of planning to wipe out Afghanistan's source of national income (opium), transform its political culture (bribery and corruption), reform its social mores (the role of women), reorder tribal power and ignore the threat from bordering states.

The Pentagon's use of the war to test its latest military kit, notably pilotless bombers, has been a disaster, ensuring that gains by soldiers on the ground are wiped out by aerial massacres that act as recruiting sergeants for the enemy. As for the anti-opium campaign, master-minded since 2001 by the British, it was well described this week by Richard Holbrooke, Obama's "Af-Pak" aide, as "the most wasteful and ineffective programme I have seen in 40 years". It was little more than a western taxpayer subsidy to the Taliban.

The good news from Washington is that Obama seems determined to stop all this. Under cover of a boost of 17,000 troops to Helmand, he hopes to suppress the violence for long enough to reach ramshackle deals with the Taliban, giving cover for withdrawal - first to Kabul and then out altogether, leaving local leaders to make some sort of peace with themselves, their insurgents and their neighbours.

This policy has mountains to climb. Any visitor to Kabul sees the air-conditioned edifices and entrenched interests of the new interventionism. Office blocks are filled with military advisers and NGOs, driving out Afghans and raising rents to the sky. Most foreigners are marooned with little to do, as few dare venture outside their compounds, let alone Kabul - a glaring deterioration of security since a year ago.

The politics swirling round Hamid Karzai, the elected Afghan president, are so fraught that he is reportedly on the brink of being toppled in all but name by a "chief of staff" compliant to American policy. Karzai, a wily survivor in a snakepit of feuding warlords, druglords and Taliban, is unlikely to go quietly. Why Nato should thus want to destabilise this last shred of Afghan democracy under the guise of seeking to root out endemic corruption in Kabul is a mystery. The parallels with America's last years in Saigon are foreboding.

Nor has the bombing by pilotless Predators ceased. Last week, America's CIA "militants" were leaking proposals for bombing the Taliban-friendly Pakistani city of Quetta in Baluchistan. The inability of Obama or his military chief in the region, David Petraeus, to stop these ventures by subordinates is a most ominous development.

By carrying operations from the border area deep into Baluchistan, America is further undermining the internal politics of Pakistan - and "defeating our objective of countering terrorism", as Yousuf Raza Gilani, Pakistan's prime minister, bluntly described the strategy. It has been so counterproductive as to suggest an al-Qaida mole embedded somewhere in Washington's high command.

Any long occupation by an invader eventually leads to a rough equilibrium of power, each component inevitably feeding the others. UN figures suggest that barely 10% of outside aid reaching Afghanistan - including £1.6bn from Britain - goes to its intended use. Most vanishes into the same power melting pot as the opium harvest and the Taliban's sources of cash in the Middle East. The idea that eager ingenues in NGO Kabul will ever create their new Sweden is fantasy.

The old maxims remain true: getting into a war is easy, getting out is hard. Obama seems to realise that the fate of America's Afghan adventure has come to depend not on what Nato does or does not achieve, but on the good offices of the emergent Taliban and the stability of the currently shambolic regime in Islamabad. In other words the balance of power rests roughly where it was before this wretched business began in 2001.

As with the Russians so with the west: this poor, intensely private country will one day see off another invader who sought to reorganise its history with guns, bombs and money. It has not worked. It was never going to work. Oh so painfully, we are now beginning to understand.'