Three different (albeit inter-related) debates are actually going on here: one about the changing nature of sovereignty; one about whether the UN authorization of force in Libya was good, bad, and/or a precedent for anything; and one about what the U.S. and other nations should or shouldn't do in the broader Middle East, most notably Syria, Yemen, and Bahrain. I'm going to focus mostly on the sovereignty debate, with some discussion of how it relates to my views on Libya. For any readers who may have missed it, I wrote a "for all you skeptics out there, look at Tripoli now" op-ed last week.
So ... sovereignty. Shelves and shelves of books have been written on sovereignty; I've written a number of lengthy and heavily footnoted articles myself, most recently in the Stanford Journal of International Law. It may also be worth noting that I made my academic name by defending states as the primary actors in the international system against proponents of what was known as the "new medievalism" back in the 1990s (when of course most of my Twitter interlocutors were barely born). But let's start with what I did NOT say about sovereignty. Contra to Joshua Foust's claim, I never said "sovereignty is irrelevant to the relationship of a society to its government," nor indeed did I say that "both governments and societies [are] 'subjects' of international law." I said that international law has come to recognize that "both governments and their citizens have rights as subjects of international law and have agency as actors in international politics," which is a very different thing. Citizens are still represented in the international system primarily by their governments, exactly as Foust argues. And governments are still the only bearers or holders of sovereignty under international law. Becoming a "sovereign nation" is still your passport to being a formally recognized player under law in the international system (think South Sudan's and Palestine's quests for statehood).
But, and it's a big but, now that individuals, who make up societies, also have rights under international law, governments can no longer exercise their sovereignty domestically without constraint. In plain English, they can no longer massacre or exterminate large swaths of their own populations without consequences under international law. This is where the writer Dan Trombly comes in. He seems to think that good international lawyer that I am, I remember my Grotius but have forgotten my Hobbes. "Slaughter insists that the origin of sovereignty was to protect a people from external threats," he writes, when in fact, "The origin of the sovereign is to protect people from external threats and each other."
Here both Trombly and Foust again elide a couple of key words in my original argument. I never wrote that the origin of sovereignty was to protect weaker states from stronger states, but rather that such protection was the most compelling reason for "the doctrine of non-intervention." The sovereign monopoly on the use of force within a state is of course the sine qua non of domestic security, as is the sovereign's control of the state's borders. That's why we worry so much about fragile states, understanding that they are often as much of a threat to international peace and security as aggressive strong states. We need strong sovereign states to stop citizen-on-citizen violence (whether from drug cartels or insurgent groups) and prevent it from spreading across borders just as much today as we did in 17th or 18th century Europe. But the experience of the 20th century taught us that we also have a problem when the sovereign uses its monopoly on force to exterminate large numbers of its own citizens for political, ideological, ethnic, or religious reasons. Sovereign states that do this turn out not to be such good neighbors, creating massive refugee flows and often fomenting regional instability. Worse still, whatever personal or political pathology leads them to think it is fine to commit genocide, crimes against humanity, or ethnic cleansing at home tends to spill over into the international arena. Human rights law came into being not simply because of the moral horror of the Holocaust but also because the world's nations realized that it would have been easier (although never easy) to stop the Nazis after Kristallnacht than waiting until the invasion of Poland. In the 21st century, as Zack Beauchamp rightly emphasizes from my original post, as many or more international security problems will arise within states as between them.