A quite interesting CFR Backgrounder on the legal doctrine of hot pursuit - originally part of the law of the seas, but now applied by countries seeking to justify cross-border raids. The backgrounder is focused on jihadist infiltration of Iraq over the Syrian border, but apples just as well to the Pakistan/Afghanistan border. It is undoubtedly a thorny legal issue:

Legal experts agree that the principle of “hot pursuit,” as it pertains to sovereign territories versus the high seas, remains unsettled. “Let’s say [U.S. forces] were to wait for a bunch of terrorists to cross into Iraq and launch an attack and then chase them over the [Syrian] border, no one will ever complain about that,” says Scharf. “But to invade another country without an actual pursuit on is going to stretch the idea of international law.”

Peter Danchin of the University of Maryland School of Law says if states want to prosecute someone for war crimes or crimes against humanity, usually they need to have them extradited. “This idea of ‘hot pursuit’ is just an attempt to twist the law of the sea doctrine into a self-defense idea. What you’re talking about is the use of force against the territory of another state,” which brings up touchy issues of state sovereignty. “Let’s say [the jihadis] go into Turkey?” he asks. “You’d have a hard time making the case that the 101st Airborne should go in and take them out without Turkish consent.” Further complicating the problem, Danchin argues, is that the United States is not the sovereign in Iraq. “It has fewer rights as an occupier than it does as a sovereign,” he says, referring to the legal use of force. David M. Crane, an expert on international law at Syracuse University, says if these foreign jihadis are apprehended on Syrian soil, they should be tried under Syrian domestic law. Any armed incursion by U.S. forces into Syria, he adds, would “ be a serious breach of international law and technically an act of war.”
Much more at http://www.cfr.org/publication/13440/.

(Additionally, the CFR just put out a report on the Pakistani Tribal Belt that I believed Westhawk referenced the other day, http://www.cfr.org/publication/16763/ )

Given the renewed attention to Afghanistan, and the calls for American troops to work over the border if necessary (as well as the revelation that the Pentagon and White House have restrained SOF commands eager to chase enemies into Pakistan), I think the legal niceties of the situation are irritating and inconvenient, but essential to the future of the US/Pakistan relationship. . . anyway, check it out for yourselves.

Regards,

Matt