On the first, empirical point: Quiggin is factually correct that interstate war has been on the wane since 1945 (though whether a lot of interstate wars were simply replaced by civil wars between state proxies is another question entirely). Asserting that this is due to the ever-growing power of international law would be a reeeeaaaaallly big stretch. There is likely no one satisfactory answer to the question. Liberal internationalists would argue that as the world has become more liberal, it has become more peaceful. The spread of democracy, the rise of economic globalization, and the empowerment of international institutions have all made war a more costly and less desirable option. Realists would provide a different explanation. They would argue that the spread of nuclear weapons among the great powers in the system has provided a powerful dampening effect on systemic international violence. Furthermore, the unparalleled military hegemony of the United States has deterred challengers from using force as a way to affect global order.

For those who believe that the cause of this decline in conflict is the growing power of international law, ask yourself the following question: if U.S. military hegemony disappeared, would you expect the outbreak of war -- and the stability of global governance -- to be the same as today?

On the second point, Quiggin is trying to frame the debate by using the Very Scary Terms "aggressive war" or "non-defensive" war. Aggressive to whom? One state's "aggressive" or "non-defensive" war is another state's "defensive" or "prudential" action.

Even under the aegis of current international law, it is pretty easy to devise justifications for a wide range of military actions. In part this is because -- with profound apologies to Alex Wendt -- international law is what states make of it. If the U.S. can't go to the United Nations to justify action in Grenada, there's always the Organization of Eastern Caribbean States. If the Security Council won't support action against Kosovo, NATO will (it's not just the U.S. -- the Warsaw Pact was useful for the 1968 invasion of Czechoslovakia, and it will be interesting from here on out to see how China uses the Shanghai Cooperation Organization). Beyond the EU, there is little to no hierarchy in international law, and there are a sufficient number of international bodies such that a state can find casus belli somewhere (again, I'm not saying whether this is a good thing or not. I'm saying that an ex ante pledge to adhere to international law doesn't work the way Quiggin thinks it does, because there's always a way to forum-shop). The days when a foreign policy leader says, "F&$k it all, I'm invading Poland!" are long gone (actually, they never existed. Even Hitler had Operation Himmler to justify the 1939 invasion of Poland under international law).

The consensus I ascribed to the "foreign policy community" is that the U.S. would not categorically rule out the use of force if its vital interests are threatened. As previously noted, there's a lot of wiggle room on "vital interests." More importantly, however, no state that sees a vital interest threatened believes that it would be waging an aggressive war if it opted for force as a policy option. And no country is going to be comfortable having, say, the United Nations as being the actor that grants them the permission slip to do something (particularly since, as Quiggin himself acknowledges, the UN's power structure is both anti-democratic and woefully anachronistic).

Does this mean international law is so protean as to be completely worthless? No. Henry Farrell has a great post that discusses different IR approaches to international law, which is well worth reading. There are instances where law can constrain state action. My position, however -- and I'd say this is likely the consensus (but not unanimous) view of IR scholars -- is that those constraints are far more powerful in the economic realm than they are in the security realm. And the reason is that the stakes are perceived to be much, much higher in the security realm, and governments are going to be risk averse on these issues (click here for the classic formulation of this point).

UPDATE: Because all current debates of this type go back to Iraq, Robert Farley makes some interesting points related to questions of defining "national interest" as well as adherence to international law with regard to Iraq:

I'm actually not sure how far the interrogation of the "national interest" concept gets us in terms of Iraq. While O'Hanlon and Pollack may have made mention of the national interest in some media fora, for the most part both of them made concrete (and wrong) arguments about how the invasion would forward some particular interest, thus avoiding the nebulous national interest justification. Indeed, I'm pretty sure that Pollack even included the furtherance of multilateral institutions as part of the reason for invading Iraq, thus suggesting that international law has a value that should be included in the US interest calculus. Some arguments for invading Iraq were quite explicit on this point, suggesting that the invasion was the only way to "save" international law and the United Nations, which was on the verge of failure because of the spiteful French.

On the whole, in fact, liberal hawks (and even some conservatives) made much more rhetorical use of international law and a sophisticated understanding of the national interest than did some opponents of the invasion. In the international relations community, "national interest" is a concept most often used by realists, who while recognizing the problems with the term still find it analytically useful. Realists, however, were among the firmest opponents of the Iraq War, which was especially notable given the fact that realists tend not to care a whit for international law or humanitarian issues.

What this all amounts to, I think, is that while the use of "national interest" as political rhetoric is full of problems, challenging the concept doesn't do much for us in the context of the Iraq War. Proponents of the war tended to make wrong, but sophisticated, arguments that invoked particular values rather than nebulous "interest", while at least some opponents (realists in the academic community, especially) held to the least sophisticated conception of national interest, but still opposed the war.

I'm sympathetic to this point, certainly, but my guess is that no laundry list provided by the candidates will ever satisfactorily answer his question. In 1949, South Korea was not thought to be in our area of "vital interests" -- until it was invaded.

Defining vital interests to U.S. foreign policy is like Potter Stewart's definition of pornography -- you know it when you see it.