The question of where the boundaries of states properly begin and end, of over whom they ought to have authority, is, a cursory glance at history will tell us, a vexed one. It strikes me it is unlikely there is any general principle by which such matters ought to be decided, given the disparate sets of moral considerations - those of legitimate expectation, of rights of self-government and to a minimal standard of living, for example - that bear on them, and still less likely that there is any such principle by which such matters can be decided. Further, there is a sense in which the question of the proper boundaries of a state are prior to questions of its internal organisation: most obviously temporally, in the sense that a democracy has to agree, in some minimal sense, on who the demos is before that body can rule.
These questions, though, are not totally divorced from one another. The same sorts of considerations bear on both of them, although not necessarily in the same way, and they clearly reflect back on each other. After all, finding that democracy requires some agreement about who constitutes the demos has considerably less bearing on whom to include in a state if you are, for whatever reason, not a partisan of democracy. Likewise, Rousseau, for example, thought that only non-commercial nations could remain free for any length of time, because trade, by undermining the robust, autarkic self-sufficiency of the citizens and creating inequality, would erode republican virtue and hence the foundations of the free state. Similarly, size or religious difference, by separating people, would prevent the sense of communal identity essential to that republican virtue from taking root.
Aristotle's claim that only Gods and monsters live outside the polis is relevant here. By letting us know who could survive outside the polis, it tells us something about both whom the polis was for, and, by inference, the kinds of goods it might be expected to provide them with. It is with this in mind that I want to probe a little more at the idea of international law. International law, like any kind of law, is a set of supposedly authoritative directives, and as such, is a political institution of sorts. I want to use this Aristotelian claim to think about whom this particular political institution is for, and what kinds of goods it might be expected to provide them with.
The first and most obvious thing to say is that what I've already said more or less commits me to the idea that the USA is, with the current distribution of global military power, in military matters, a God or a monster. As I said in the comments to the previous piece, "the point of being a hegemon is that you are in an elite of one". Aristotle's claim, I think I am right in saying, is perhaps best interpreted as something like a statement of identity, resting on some quasi-naturalistic teleology: we identify something as a (hu)man, and hence a possible citizen, because it cannot survive outside the polis, outside some form of regulation, because it needs those rules. Conversely then, an entity which doesn't need those rules, which can flourish or at least survive without them, like, in terms of its military capacity, I think the USA can, is not a possible citizen: it is not suitable for the goods of the polis.
The more interesting area is what that kind of view commits me to in terms of the other areas of international law, and particularly on issues like cosmopolitanism and global justice. Thomas Pogge, whose work I probably ought to know more about, has, as I understand it, been trying to adapt a Rawlsian view to these questions for some time. The idea, again as I understand it, is that, contra Rawls' own view in 'The Law of Peoples', the various ad hoc and more deliberate institutions of the global economic order constitute a basic structure of a sort - perhaps not the same kind of basic structure that the legal and social institutions of a state do, but a basic structure nonetheless. This matters because the existence of a basic structure is what calls for a theory of justice: because the set of institutions that make up a basic structure significantly determine individuals' life prospects, they are required to be just. Thus, in virtue of this structure, we have duties of justice to the citizens of other states, duties the shape of which is likely to be determined by the features of that structure.
In terms of the Aristotelian maxim, it looks like saying that, because citizens of other states are already in certain respects citizens of our polis - they live under rules which we have some hand in setting up, determining and administering, rules which make a substanial difference to their lives - we ought not to treat them as Gods and monsters. Of course, to fall into the category of God or monster is not to fall out of the scope of morality altogether: it is rather to invite a different style of moral assessment or action, one distinct from that called for by a set of shared institutions. As Bernard Williams puts it - you knew it'd come back to him, surely - whilst discussing slavery's, and particularly that of the Helots in Sparta, failure to be a political institution:
[t]here can be a pure case of internal warfare, of the kind involved in the case of the Helots... [and]... [w]hile there are no doubt reasons for stopping warfare, these are not the same reasons, or related to politics in the same way, as reasons given by a claim of authority.
The shared institutions of the global economic order, though, do involve a claim of authority: they do not rest on mere force, but some kind of quasi-consensual arrangement which is supposed to, in some way, justify them. As such, they are political, and thus require legitimation in the usual, political, sense.