The attack on the Marriott hotel is a shocking blow to Pakistan. What is really frightening is that the security measures in place worked: the lorry was stopped by guards at the barrier outside. As I found during visits to the Marriott during my recent stay in Pakistan, they were vigilant and effective.

Against a tonne of explosives, however, there is not much that can be done – except to cordon off the entire neighbourhood. Most of the Western embassies and consulates in Pakistan are protected in this way, but the Marriott, like other international hotels, government buildings and media offices, is on a main road that cannot be blocked without causing massive disruption. If the Islamist terrorists have matériel for many bombs of this type, protecting such targets will be impossible. If there are enough volunteers for suicide terrorism any public figure who condemns the extremists will be in mortal danger. The Pakistani State, with its overstretched, demoralised and desperately underfunded police, just does not have the resources to defend against a threat on this scale.

This is the real threat in Pakistan: not imminent “Islamic revolution”, state failure or loss of control of nuclear weapons, but a spread of terror, with sooner or later – as in Algeria and elsewhere – the State in desperation resorting to counter-terror in response. The State will survive but the bloodshed could dwarf anything seen since the loss of East Pakistan.

One effect will be to increase the stream of educated people leaving the country. In Peshawar many business and professional families are preparing lines of escape to Western countries. If this occurs across Pakistan, accompanied by a flight of foreign businessmen and foreign investment, economic growth will vanish, further fuelling extremism.

The willingness of the Pakistani Taleban (the Tehrik e-Taleban e-Pakistan, or TTP) to carry out massive terror attacks in Pakistan itself has been obvious since last month, when it killed dozens of workers at the munitions complex at Wah. I expected this to produce a strong reaction against them in the Pakistani public, but in the Pashtun areas at least this was not the case. All too many of the people I spoke to there were prepared to justify the attack because “this was a military target and the military are taking American money to kill their own people”, as one farmer told me.

Also of importance in sapping the population’s will to support tough action against the TTP is the extent to which conspiracy theories dominate much Pakistani popular and even elite thinking. Again and again I heard that the Wah attack was not the work of the TTP (even though they publicly claimed responsibility) but of the Indian, Israeli or US secret services.

The latest attack, however, may produce two good results. The first is a stronger determination in the civilian Government and the military to work together to defeat the extremists. The second is a reminder to the West that what happens in Pakistan is not just an aspect of our campaign in Afghanistan. Pakistan is a much more important country and is in serious peril. It needs all the help we can give.