Leading article: The case for withdrawal from Afghanistan is not yet made

Yesterday was a black day for the British Army and the Western mission in Afghanistan.

The appalling slaughter of five soldiers in Helmand – killed in their base by a rogue member of the Afghan security services – is likely to undermine public support for the mission here in Britain. But how should it influence policy-makers? A single attack must, of course, be seen in its wider context. Yet that wider context is looking increasingly bleak. This atrocity comes in the wake of Hamid Karzai's disputed presidential election victory and a growing feeling that his regime is corrupt and illegitimate.

The attack also strikes at the very heart of the Western strategy in Afghanistan, which is to build up the domestic police and military and allow Afghans to take over responsibility for their own security. The West's objectives for Afghanistan are looking increasingly unrealistic, and evidence is mounting that the foreign military presence in the country is doing more harm than good.

So has the time come to withdraw? This newspaper supported the Western intervention to topple the Taliban in 2001 after the regime refused to expel the leaders of al-Qa'ida who had masterminded the 11 September terror attacks. And we have backed Western efforts in the years since to help rebuild the shattered Afghan state. Yet circumstances have plainly changed in recent years. And the case for withdrawal has grown considerably stronger.

However, the case is not yet overwhelming; not least because no convincing alternative strategy for protecting Western security interests in the region has been put forward. We need to consider the consequences of letting the Afghan government face the growing Taliban insurgency without Western military assistance. There is a significant risk that the Taliban would return to power. And such an ideologically driven regime might well decide to host al-Qa'ida once again.

Some appear to believe that such a threat would be manageable. Kim Howells, the chairman of the Commons intelligence and security committee, argued this week that British resources should be channelled to strengthening our domestic border controls and building up our intelligence networks. And the US Vice-President, Jo Biden, has reportedly proposed a stripped-down Western counter-terrorism strategy in Afghanistan, based on remote drone attacks on terrorist operations in the Afghan-Pakistan border region. Domestic intelligence is, of course, vital. But the notion that Britain can manage its security threat entirely from within its own borders is unconvincing.

As for stepping up drone attacks, these weapons are already causing heavy civilian casualties and provoking popular anger in Afghanistan and Pakistan. Such strikes might succeed in eliminating terrorist targets, but the problem is that, in the long term, they merely feed the problem of Western resentment.

Then there is Pakistan. In the event of a Western withdrawal from the region, the Pakistan military might well decide to rekindle its old alliance with the Taliban across the border and make peace with its own domestic jihadists. That would be disastrous for Britain's security interests.

Unlike Iraq, Afghanistan was not a "war of choice". And the Pashtun tribal areas are still at the centre of global terror networks. Those advocating Western military withdrawal from Afghanistan need to do more than simply urge a rush for the exit. They need to provide a realistic replacement strategy for protecting Britain's national security and promoting stability in this most dangerous of regions.