IT has been likened to a child learning to ride a bike.

At some point the stabilisers have to come off. Of course, the timing involves a calculated risk and getting it wrong can mean tears, recriminations and bloody knees. Today David Cameron will announce “a modest reduction” in Britain’s fighting force in Afghanistan by the end of next year. It is likely to amount to about 500 soldiers, on top of the 450 being withdrawn this year. However, despite grand-sounding plans for future aid and an Afghan version of Sandhurst, the commitment to withdraw all troops by 2015 was reiterated yesterday. Come what may, the stabilisers are coming off.

The extent of the risk was illustrated poignantly during the Prime Minister’s two-day visit. A walk-about in Lashkar Gah, intended to show how safe it is in the capital of Helmand province these days, had to be called off, basically because, after the death nearby of a soldier from the 4th Battalion Royal Regiment of Scotland, it was considered too dangerous. Another flag-draped coffin effectively undermines the message of hope he was hoping to send out. Yet the town is due to be handed over to the control of Afghan forces this month. Security has improved in some parts of Helmand, notably Nad Ali, but when Mr Cameron rises to make his statement in the Commons today, he will have the words of military commanders ringing in his ears, including the insistence that British troops need two more fighting seasons to get the job done. Otherwise, what has been achieved so far in terms of stability and territory won will not be sustainable.

It will not help him in a General Election year if the Taliban marches into Kabul the moment the last Union flag is lowered at Camp Bastion.

The second big issue is about Afghan political stability and governance, issues largely beyond our control. For the Taliban this conflict was always more about Pashtun nationalism than spreading global jihad. Though President Hamid Karzai is himself a Pashtun, that grouping lost ground in the recent elections and now the fragile inter-ethnic coalition around him looks distinctly shaky. The brazen Taliban assault on the Inter-Continental Hotel in Kabul was a psychological blow and the flight into exile of the governor of the fraud-ridden Kabul Bank does not bode well.

Afghanistan’s best hope lies in persuading the more amenable of the Taliban into swapping guns for ballot boxes. Urging that course, Mr Cameron’s message to them yesterday was: “You are losing this fight.” Unfortunately, it probably does not seem that way to them, as arms and reinforcements continue to pour in across the porous border with Pakistan. In the Afghan army, quantity is no substitute for quality. Nevertheless, a perpetual stalemate is in nobody’s interests and Afghan troops may perform more effectively when they know they do not have British and American forces to fall back on. Measured withdrawal is the least worst option.