The diplomatic relationship between Pakistan and the US – testy and fraught with mutual suspicion – bears out as few others the dictum of Lord Palmerston's that countries have no permanent friends or allies, only permanent interests.
Yesterday's release of Raymond Davis after "blood money" payments to the families of the victims reflects not a sudden flowering of mutual affection between the two governments but a shared understanding that a failure to resolve the case of the CIA operative would have been a disaster for both sides.
Whether they like it or not, the US and Pakistan are condemned to work together – or at least, to try to. Without the support of Washington, Pakistan would be more isolated than ever, and deprived of a major source of financial and military aid.
For the US, Pakistan is crucial to its geostrategic interests in the region. It is key to any hope of a lasting peace in Afghanistan, and its co-operation is vital if the US is to capture the al-Qa'ida leaders who have fled to Pakistan's remote tribal areas.
Its role as a nuclear power (and suspected nuclear proliferator), its links to Islamic extremism, its corrupt and fragile politics, all combine to make Pakistan one of the most dangerous places on earth. For all these reasons, the US could not ignore it, even if it wanted to. And so the relationship somehow staggers on, despite a level of mutual distrust that in other times and other circumstances would make them enemies.
Pakistan often feels humiliated, treated as a client state to whose sensibilities the US pays no heed, as it conducts cross-border raids and drone attacks on supposedly sovereign Pakistani soil, and deploys armed CIA operatives like Mr Davis inside the country, claiming they are above the local law.
It also feels betrayed. During the Cold War, the US was aligned with Pakistan as the Soviet Union courted its great rival, India. Now Washington is seeking to strengthen its ties with Delhi – and that tilt, Islamabad believes, can only be to the detriment of its ties with the US. Small wonder, therefore, that anti-Americanism is rife on the streets of Pakistan's cities.
America, for its part, has trouble believing a single word it hears from the Pakistani government and military. The latter promises co-operation, but offers succour and sanctuary to US foes in Afghanistan. Elements of Pakistan's powerful intelligence service are sympathetic to the Taliban, while Pakistan is emerging as a centre of Islamic terrorism in its own right. As for American aid, much is lost to corruption, while much US military assistance merely goes to beef up Pakistan's defences against India.
Two developments might place the relationship on a firmer footing. One is the inclusion of the Taliban in negotiations on the future of Afghanistan, which seems to be starting to happen. The other is a transformation of Pakistan's attitude to India, which is less likely. Failing both, the US/Pakistani relationship is likely to stagger on, from one crisis to another.