After the arrest of the CIA officer Raymond Davis in January, and the secret raid that killed Osama bin Laden in May, it was hard to imagine that relations between the US and Pakistan could get any worse. Alas, the Nato air raid at the weekend close to the murky border region with Afghanistan, in which at least 24 Pakistani soldiers died, has managed to achieve that feat.
Details of the incident are unclear. Islamabad maintains the attack was unprovoked, while Nato says the strikes were called in after US and Afghan forces themselves came under fire from the Pakistan side, where Taliban units and other Islamic extremists notoriously operate. What is clear is that there is a genuine risk of rupture between the two countries.
It was always assumed that the common interest of both sides in working together would prevail, whatever the tensions of the moment. For Washington, co-operation with Pakistan is essential for a negotiated political settlement that will permit the US to withdraw its troops from Afghanistan by the target date of 2014. Meanwhile, the closer the US can watch the security of the nuclear stockpile of a highly unstable country, the better. For its part, Pakistan and its powerful armed forces rely heavily on American aid.
But this underlying assumption is no longer automatic. Distrust between the two sides is now almost total. The retaliatory measures taken by Pakistan, most notably the closure of Nato supply routes to Afghanistan, reflect a popular fury at this latest, and deadliest, example of apparent US contempt for the country's national sovereignty that its government could not ignore, even if it wanted to. For their part, though, many officials in Washington believe barely a word they are told by their counterparts in Islamabad, and least of all on issues of counter-terrorism. This is a recipe for rupture.
Somehow the relationship must be patched up, and not constantly held prisoner to incidents along a remote border that no one controls. For all the reasons listed, a break between the US and Pakistan would be a strategic disaster, with implications stretching well beyond the immediate region.