By Paul Reynolds
World affairs correspondent, BBC News website
More than 130 people have been killed in a suicide attack on the convoy of former Pakistani Prime Minister Benazir Bhutto, following her return to Pakistan after eight years of self-imposed exile.
The attack on Benazir Bhutto is another example of why Pakistan is regarded with so much trepidation by Western diplomats.
Ms Bhutto said she was returning to Pakistan to combat extremism
Although Pakistanis always argue that its instability is not as bad as the outside world thinks, and that extremists will never take over, those extremists do have the ability to cause mayhem.
The return of Ms Bhutto was supposed to shore up the position of America's ally General Pervez Musharraf. That was why both Washington and London supported this move.
The general would continue to be in overall charge as president. Benazir Bhutto would provide greater stability and respectability as prime minister. That was the plan.
Her return has shown instead how unstable the whole edifice is.
Generations of militants
The nightmare in the West is that the Taleban and its supporters, already entrenched in the tribal areas of Pakistan and fighting a war in Afghanistan, will exert a permanent and destabilising influence on Pakistani politics.
Only this summer, Pakistani forces stormed the Red Mosque, which had been taken over by militants.
It is thought that Benazir Bhutto's support for action against Osama bin Laden was one of the reasons that threats were made against her - and probably the reason for the suicide attack.
Washington had once hoped that the war to remove the Taleban from Afghanistan after 9/11 would lead to peace. It might have done for a brief period but now the problem is back.
But Pakistan's crises go wider that the Taleban and al-Qaeda.
Pakistan has never managed to develop a settled, democratic form of government, but has always been subject to the violent swing of political pendulums.
It is never far away from violence.
Suicide attacks have increased since troops stormed the Red Mosque
It was closely involved in the war against the Soviet Union in Afghanistan. It has fought wars with India over the issue it was born with and still faces - the dispute over Kashmir.
This has bred generations of militants, who have launched attacks within Indian-controlled Kashmir and major Indian cities.
Militancy with a Pakistani connection has also been evident in Britain.
The British Foreign Secretary David Miliband, condemning the attack on Ms Bhutto, noted that 70% of the plots in Britain were linked to Pakistan.
And nobody forgets that Pakistan has nuclear weapons.
These have been the source of further trouble, because their creator, AQ Khan, secretly gave away his nuclear secrets in defiance of the international treaty against the proliferation of nuclear weapons.
Ms Bhutto hoped that by going back, she could change all that and restore Pakistan to a democratic path. But her way has now been blocked.
And nor can Pakistan withdraw from international politics and try to concentrate on facing up to its own internal challenges, in which the economic disparity between rich and poor continues to feature strongly.
General Musharraf is seen by the United States as the rock on which its policies in the region are based. It is in the US' interest to see him continue in power, one way or another.
For Pakistan, there is no escape from its place at the centre of the post 9/11 world and its cockpit role in the US-declared war on terror.