It’s not clear what led to NATO strikes on two Pakistani border posts this weekend, but there can be no dispute that the loss of lives is tragic. At least 24 Pakistani troops were killed. We regret those deaths, as we do those of all American, NATO and Afghan troops and Pakistani and Afghan civilians killed by extremists. Washington and Islamabad need to work together, urgently, to ensure that this incident does not destroy their deeply troubled relationship. The United States needs Pakistan’s cooperation — as grudging as it is — to pursue the fight against the Taliban. And without American support, Pakistan’s fragile government will be even more vulnerable to extremist attacks. So far, Pakistan’s leaders seem most interested in fanning popular anger. The Obama administration and NATO have wasted precious time, allowing the crisis to escalate.