U.S. commanders had boasted in recent months about the success of what they like to call the “Kabul security bubble.” Afghans forces were taking the lead in this effort, the commanders argued, and were showing that they could keep the insurgents away from major targets. That transfer to Afghan control is the linchpin of the U.S. strategy — and Kabul is supposed to be the showpiece. It’s a bit harder to argue after today’s attack that it’s going well.

Because of the critical role of Kabul, U.S. commanders have structured their campaign plan around protecting the capital. In the districts between Kabul and the Pakistan border, known in NATO-speak as Regional Command East, or simply “RC-East,” the strategy calls for “layered defense” along the routes traveled by Taliban fighters — with the goal of stopping them before they get to Kabul.

But today’s assault on a symbol of British power — and by extension, the NATO mission — shows that the layers of defense are still porous. It’s an important psychological blow, since the overall coalition mission in Afghanistan depends, in significant measure, on nongovernmental organizations based in Kabul, which can’t function without reasonable security. Indeed, U.S. commanders view Kabul as more important to Afghanistan’s security than Baghdad was to Iraq’s.

An earlier warning of Kabul’s continuing vulnerability was the Taliban’s June 28 attack on the Intercontinental Hotel. But in that case, U.S. commanders argued that although the Taliban had managed to gain entrance to the hotel compound, Afghan forces had fought back hard (assisted by U.S. helicopters and other support). The news headlines may have stressed the shock of the hotel attack, but coalition commanders privately took a measure of comfort in the Afghan response.

It will be harder to see a similar “silver lining” in the assault on the British Council. This was a fortified, high-value target in an affluent western district of the city. The attack was a well-coordinated, two-phase assault: First, at about 5:30 a.m., two car bombs shattered security at the entrance; then suicide commandos raced into the building on foot. The compound wasn’t secured until more than six hours later.

Like the April 25 prison break in Kandahar and the assault on the Intercontinental Hotel, this was the kind of high-visibility operation at which the Taliban remains adept, despite all the firepower of the U.S.-led coalition. U.S. officials can say it’s a psychological more than a military success. But that’s just the point. If Kabul’s nerves are jangled much more, the U.S. strategy is going to begin to move in reverse.