The best guide to what the shape of things might be post-US withdrawal comes from Afgantsy, a brilliant book on the Soviet experience in Afghanistan by the former British diplomat Rodric Braithwaite. I have written of this book before, but not of its treatment of Afghanistan after the Soviets left.

Russian troops left in February 1989, but it took more than three years for Afghanistan's puppet government, led by Mohammed Najibullah, to fall. This is because the Russians gave Najibullah $3 billion in aid a year, as well as weapons and ammunition.

Najibullah, like Harmid Karzai today, had 300,000 troops at his disposal. As Braithwaite shows, these troops fought well and effectively against the mujaheddin while ever Soviet aid kept flowing. They did so in part because of the savagery of the mujaheddin. There was no incentive to surrender. When the mujaheddin finally took custody of Najibullah himself, they tortured and castrated him and hung his body in Kabul's square.

Rather than the mujaheddin causing the collapse of the Soviet Union, it was the collapse of the Soviet Union that led to the cessation of aid to Najibullah that led to his defeat.

The pattern of the mujaheddin's military success was that the Pakistanis backed them in the south especially, the Pashtun area, while Najibullah's area of effective control gradually receded to Kabul and its surrounds.

After the mujaheddin warlords fell out with each other, the Pakistanis used the Taliban to take control of Afghanistan.

The basic pattern will surely reassert itself. The Americans will provide billions in continuing aid to Karzai or his successor, as well as perhaps some special forces as a Praetorian Guard, and will be able to maintain some kind of government in Kabul.

The Pashtun south will be dominated by Pakistani proxies and allies, some amalgam of the Taliban, perhaps marginally rebadged.

This basic pattern will play out whether we leave next month, next year or in 2014. We should get out as soon as possible and then, from a distance, try to help what friends or collaborators we have, in Afghanistan and, much more importantly, Pakistan.

The US now has the capability to strike any terrorist training camp that emerges in Afghanistan. That pretty much satisfies its strategic needs. In the meantime, Pakistan is going to hell.

It is abundantly obvious that the US and allied presence in Afghanistan is making things infinitely worse in Pakistan. The US understood an analogous problem a decade ago in relation to Saudi Arabia. US troops were necessary to secure it from Iraq, but the presence of US troops on Saudi soil was politically and socially unsustainable. So the US withdrew before the US-Saudi relationship fell into absolute crisis.

This all points to a long-term need to engage Pakistan, on economic reform and, above all, on keeping its nuclear weapons safe. Australia does this now by giving aid to Pakistan and training its soldiers in Australia.

But surely this is completely unacceptable while the Pakistani military is assisting the terrorists who murder Australian soldiers in Afghanistan.

Only after we leave Afghanistan can we decently engage the Pakistani military. Otherwise, we are helping an institution that helps others to kill our soldiers. Whatever the strategic imperatives, that price is too high.