Image caption Relations between Pakistan and the US have been strained

The death of al-Qaeda leader Osama Bin Laden near Islamabad has important implications for relations between Pakistan and the US.

Pakistan has been the epicentre of the battle against al-Qaeda in its global jihad.

It is the West's most important ally in this struggle and, so far as the CIA is concerned, it has also proved to be the most difficult ally.

The Pakistanis have consistently denied ever having any links to al-Qaeda or their former hosts in Afghanistan, the Taliban, whose leadership has had sanctuary in Pakistan since 2002.

And yet the discovery that Osama Bin Laden had been living in a large, custom-built compound within a miltary zone that includes Pakistan's military academy once again raises an obvious question.

What did the intelligence arm of the Pakistan military - the Inter Services Intelligence Directorate (ISI) - know and when did it know it?

The compound is reported to have had high walls, barbed wire and security cameras. Who built it? Did none of the local authorities, including the police and the military academy, ever have their suspicions?

Crisis point

Following 9/11, Pakistan did arrest Khalid Sheikh Mohammed, alleged to have been the "principal architect" of the attack.

The US believes that although al-Qaeda has been largely driven from Afghanistan, the job of hardening Afghanistan against a return of al-Qaeda has yet to be completed

But since 2005, the US authorities have become increasingly persuaded that the ISI has taken an "a la carte" approach to Islamist terrorist groups - picking and choosing which group to support.

The choice has depended on how the ISI has seen each group serve its national interest.

Crudely, this boils down to whether they act as a hedge against the growing regional influence of Pakistan's long term foe - India.

If the US assessment is that there were elements within the ISI who did know where Osama Bin Laden was hiding and, moreover, did nothing about it, US relations with Pakistan - already severely strained - will reach crisis point.

Billions in aid is sent every year to Pakistan by the US, UK and other Western countries.

Sanctuary

The fact that Osama Bin Laden has apparently been living for years under the nose of the Pakistan military also revives the question that has increasingly dogged the US-led coalition in Afghanistan: Why are we still fighting in Afghanistan when it is Pakistan from where the Taliban insurgency is being directed?

Both the Pakistanis and the US have said Pakistan was not given advance notice of the raid.

The official Pakistan response was low-key, saying only that Osama Bin Laden's death "constitutes a major setback to terrorist organizations around the world".

That it took the Pakistani authorities more than 15 hours before making their public response may have been out of a desire to distance themselves somewhat from the operation.

Drone attacks

The extent to which Pakistan has approved specific US requests for assistance against al-Qaeda and other extremists is one of the key tests President Barack Obama set in assessing this July whether he can safely start to withdraw troops from Afghanistan.

Pakistan has allowed the US secret bases in Pakistan and to launch drone attacks against specific targets. These have more than tripled under President Obama.

However, the US believes that although al-Qaeda has been largely driven from Afghanistan, the job of hardening Afghanistan against a return of al-Qaeda has yet to be completed.

The Taliban regards the government of Afghan President Hamid Karzai as a "puppet" of the West and have demanded all Western forces leave.

There are now two main al-Qaeda offshoots which have found sanctuary in the tribal lands of Pakistan's border with Afghanistan.

The Haqqani network is responsible for many attacks against soldiers in Afghanistan and suicide bombings there, and Lashkar-e-Taiba carried out the multiple attacks in Mumbai, India, in 2008.

The US believes that as long as the Afghan Taliban leadership and the Haqqani network have sanctuaries in Pakistan, the insurgency inside Afghanistan is likely to continue.

There is also the Pakistan Taliban - against which the Pakistan authorities have moved because it has attacked Pakistani targets in pursuit of its goal of an Islamist government - and the Afghan Taliban, against which the Pakistan authorities have barely moved, and whose fighters the CIA believe the ISI have helped train.

John Ware will report and present the first of three documentaries about the decade-long Afghan conflict for BBC 2 later this summer