Analysis of the legality of the UBL operation continues to proliferate. Among the many I've seen today, I would recommend Gabor Rona's fine piece defending the legality of the attack from the point of view of the "organized armed group/continuous combat function" model (and also explaining why UBL was not "hors de combat" when he was shot). But I'm not posting now to rehash the IHL/IHRL debates. Rather, I want to highlight the jus ad bellum/UN Charter issue that really has Pakistan's attention.
Sure, the US has been using force in Pakistan for some time now, in the form of drone strikes. This has not presented a serious Article 2(4) issue because (i) Pakistan seems in fact to have consented to the drone strike program (notwithstanding public statements to the contrary issued for domestic political consumption) and (ii) in any event the strikes have been confined to border regions in which any claim to authority to use force without consent based on Article 51 would be bolstered by a highly-persuasive argument that Pakistan does not actually have the capacity to control those regions. Put simply, permitting the drone strike program does not create a clearly on-point precedent that India might cite should it decide to mount a similar clandestine raid in other areas of Pakistan against, say, Lashkar e-Taiba personnel.
The UBL operation, in contrast, does seem closer to that scenario. It was, therefore, inevitable that Pakistan would (i) purport to have ratified the operation by post-hoc consent, (ii) object that the operation was an unlawful intrusion into its sovereignty at the time (presumably on the theory that it was in fact capable and willing to deal with UBL if only the US had told it of the situation), or (iii) both. Though at first it seemed Pakistan was going to go with option (i), we are now seeing more of either option (ii) or (iii). From All Things Considered last night:
MCCARTHY: Pakistan registered its, quote, "deep concern and reservations" about the Americans' unilateral raid on Pakistani soil, saying it shall not serve as future precedent for any state. It was a nuanced message for the international community, particularly its archrival, India, whose air force chief said New Delhi also has the capability to carry out such surgical strikes against terrorists, but he didn't elaborate.
Retired General Rashid Qureshi says the Americans have sent a very bad signal.
Mr. QURESHI: Let me say that the actions that the U.S. has taken would encourage any military establishment which is against Pakistan to consider such an action and think that they might be able to do it.
Of course, India is constrained from pursuing the logic of this parallel by the very real possibility that such an operation would precipitate an international armed conflict between the two states. But make no mistake; Pakistan surely has India in mind when it contemplates its response to the UBL operation, and this will color the Pakistani response to any future US strikes outside the border region.