Leading article: Another blow to trust in Afghanistan

The death of seven CIA officers in a suicide bombing at a remote and highly secret base in rugged and militant-infested Khost Province is a reminder of both the importance of the intelligence agency in America's expanding war in Afghanistan, and of the extreme treachery of the human terrain on which it must work.

The killings were confirmed yesterday by the CIA chief Leon Panetta. But exact details of what happened are still murky. The death toll may rise, since six other Americans were wounded in the attack. Yet it is clear that the bomb was carried by an Afghan soldier trusted enough to be allowed into the base's gym area where the bomb was detonated.

The incident can only further undermine the trust that is essential if the Obama administration's strategy of training the Afghan army to take over responsibility for its country's security is to succeed. It must be more doubtful than ever that the Afghan authorities have the ability to screen soldiers supposedly fighting alongside Americans from Taliban infiltration. That worry is even greater if, as claimed by a Taliban spokesman, the bomber was not a lowly recruit but an officer in the Afghan army.

A common image of the CIA in Afghanistan is of an unprincipled private army answerable to the President alone, whose main pre-occupation has been with running "ghost prisons" where suspected terrorists were subjected to waterboarding and other tortures. In fact, the agency has played a major role in counter-insurgent operations, mounting the drone attacks against Taliban and al-Qa'ida operatives along the border. Khost Province directly adjoins the Waziristan region across the border in Pakistan, a Taliban stronghold and a possible hiding place of Osama bin Laden.

If Washington and its allies are to win a guerilla war where flexible and unconventional tactics are required, the CIA's expertise and experience will be especially precious. By any measure the losses incurred at Base Chapman represent a significant setback in Nato's efforts to prevail where British, Soviet and other foreign invaders have invariably failed in the past.