There are conflicting reports about precisely which group, or groups, were responsible for the multiple attacks in Kabul on Tuesday. Some blame the Taliban, some al-Qa'ida, yet others another group linked, if tenuously, to both. In the end, though, the exact designation of the perpetrators is less important than the fact that such a broad assault could be perpetrated so close to the centre of power.

The targets included the US embassy, Nato headquarters and several police buildings. And while 11 of the attackers were killed, so were 11 civilians and five Afghan police. It took Afghan and US forces more than 20 hours to bring the assault to an end, and culminated in a fire-fight inside a high-rise building that overlooks the diplomatic quarter. Helicopters were brought in before order was restored.

The most positive gloss that can be placed on this episode is the one voiced by the US ambassador, Ryan Crocker, whose embassy was hit by rockets. He said that the missiles had been fired from a distance, represented harassment rather than a serious attack, and demonstrated the weakness of the opposition forces, not their strength. It might be added that the absence of a large-scale terrorist attack, in Afghanistan or anywhere else, on the 10th anniversary of 9/11, supports the idea that, with Osama bin Laden dead, al-Qa'ida is now much diminished.

For all the ambassador's commendable sang froid, however, it is hard to share his view. Al-Qa'ida may be weakened, but the same cannot be said of the Taliban. US casualties have increased in recent weeks, and there is clearly enough opposition, including armed opposition, to President Karzai and the foreign forces that sustain him, to breach the capital's defences and place Kabul on a siege footing for almost 24 hours. What is more, Afghan police and security forces could not have repelled the attacks without US assistance.

The events of Tuesday may have been no more than a show of strength by just one faction of the armed opposition. But they show how fragile security remains, even in the Afghan capital, and offer a warning of how swiftly the situation could deteriorate. None of this bodes well for a smooth withdrawal of US forces by the end of 2014, but any backsliding on President Obama's timetable could make things a great deal worse.