John Quiggin says he's
. . . mildly surprised to find myself in partial agreement with Megan McArdle who notes “the wars that don’t happen in the Middle East, or Central Europe, because all the participants know that it would be a foolhardy invitation to US intervention”. The fact is that the (admittedly selective) enforcement of the interantional law against aggressive war, in which the US has taken the leading role, has had a significant effect in securing adherence to that law. But even without any obvious threat of US intervention, a great many states have abandoned the idea of military force as a legitimate instrument of public policy except in the context of (individual or collective) self-defence and UN-authorised peacekeeping and humanitarian operations. In particular, outright invasions of one country by another, with the objective of either annexing the target country or installing a puppet government, have been quite rare in the period since 1945. So the claim that international law is a dead letter is far from obvious.
This is because we don't agree. In fact, since 1945, "international law" has been enforced when some sufficiently powerful state felt strongly enough about it to do something, for various reasons ranging from post-colonial nostalgia to anti-communism, to strategic commodity security. To the extent that border disputes may have been restrained in those regions, they are restrained not because nations think "the fiery sword of UN legal justice will descend upon us" but because people think "the US doesn't like us going to war against our neighbors".
Obviously, all laws are only partially enforced, but at some point, it becomes meaningless to call this the rule of law; a law that is enforced only when the police chief feels that it will personally benefit him or someone he happens to like is not a law, it is an autocracy.
Moreover, I'm certainly no expert on the matter, but it's far from clear to me that if the UN charter were actually operational, such partial peacekeeping would even have happened. To the extent that countries are afraid of US intervention, they're afraid precisely because they know that China and Russia don't hold a veto.