Whisper it quietly. Contrary to popular opinion, the west has won the war in Afghanistan.

How do I know this? Because Barack Obama says the aim of the war is to "disrupt, dismantle and defeat" al-Qaida in Afghanistan – a strategy endorsed by our very own Gordon Brown. If that's the case, then let me spell it out to the president and the prime minister: there are no Afghans in al-Qaida, and no al-Qaida in Afghanistan.

So why not declare victory and bring the troops home?

That's not just my humble view – that's the view of one of the world's leading counter-terrorism experts, Dr Marc Sageman, of the Foreign Policy Research Institute in Philadelphia:

We've won. It was critical, after 9/11, that we went into Afghanistan to destroy the terrorist training camps that the plotters had attended … and we've done that: there are no camps left in Afghanistan, and all of the terror plots now come out of Pakistan.

Dr Sageman has impeccable credentials: a forensic psychiatrist, sociologist and scholar-in-residence with the New York police department, he served as a CIA case officer in Islamabad in the late 1980s, working closely with the Afghan mujahedin. His most recent book, based on an analysis of more than 500 terrorist biographies, convincingly argues that Bin Laden and his ilk have ceased to function as an organisational or operational entity and that the "present threat has evolved from a structured group of al-Qaida masterminds, controlling vast resources and issuing commands, to a multitude of informal local groups trying to emulate their predecessors by conceiving and executing operations from the bottom up. These 'homegrown' wannabes form a scattered global network, a leaderless jihad."

Earlier this month, in both oral and written testimony at a congressional hearing on the war, convened by Senator John Kerry's foreign relations committee and sadly ignored by much of the mainstream media, Dr Sageman demolished many of the myths and claims that have infused, confused and distorted the debate over Afghanistan.

First, the claim that fighting a war in Afghanistan protects the streets of New York and London from terrorist attack. The crux of Dr Sageman's argument, and empirical research, is that, since 2002, there has not been a single terrorist plot in the west that can be traced back to Afghanistan. "The few that have any link to a transnational neo-jihadi terrorist group are linked to Pakistan," he told me. These include the 7/7 attacks and the more recent liquid bomb plot – in fact, as Gordon Brown himself conceded in December 2008, three-quarters of the terrorist plots investigated by British authorities can be traced back to Pakistan – and not Afghanistan.

Second, the claim that a resurgent Taliban poses a threat to the west. Dr Sageman is adamant that the prospect of "deeply divided" Taliban forces retaking Kabul and returning to power in Afghanistan is "not a sure thing". Nor would a Taliban return to power "mean an automatic new sanctuary for al-Qaida." The relationship between the two organisations, he says, "has always been strained … indeed, al-Qaida has so far not returned to Taliban controlled areas in Afghanistan." It is a view shared, incidentally, by a senior member of the Obama administration, the national security adviser, General James Jones, who told CNN that "the al-Qaida presence [in Afghanistan] is very diminished. The maximum estimate is less than 100 operating in the country. No bases. No ability to launch attacks on either us or our allies."

Third, the claim that Afghanistan will benefit from an Iraq-style "surge" of western troops. This was Sageman's testimony on Capitol Hill:

Let me answer that with an old Middle Eastern proverb. 'It's me and my brother against my cousin. But it's me and my cousin against a foreigner.' So if we send 40,000 Americans ... that will coalesce every local rivalry; they will put their local rivalry aside to actually shoot the foreigners and then they'll resume their own internecine fight ... Sending troops with weapons just will unify everybody against those troops, unfortunately.

Dr Sageman is keen for policymakers in the west, who promote falsehoods and myths about Afghanistan while sitting "several thousand miles from the war zone", to acknowledge the futility of escalation, instead of recognising the success in ridding Afghanistan of al-Qaida, as long ago as 2002, and now switching the focus to Pakistan. He has another useful ally in the White House, in the form of the vice-president Joe Biden, who has been pushing in recent weeks to divert resources from Afghanistan to Pakistan and reformulate the terrorism problem as "PakAf", not "AfPak". But our own prime minister's decision to send a further 500 troops to the killing fields of Helmand flies in the face of such thinking. Dr Sageman is perplexed. "The problem is in Pakistan," he tells me. "But that's not where we are sending troops to. We're sending them to the nation next door."

The question is: why? As the military and civilian deal tolls continue to rise inexorably, in a conflict that is about to eclipse Vietnam in its length, it is high time that our politicians, generals and spies wake up to the fact that we are fighting the wrong war, in the wrong country.