Kagan insists that the U.S. and its allies are responsible for the reduction in armed conflict and violence worldwide:

If there has been less aggression, less ethnic cleansing, less territorial conquest over the past 70 years, it is because the United States and its allies have both punished and deterred aggression, have intervened, sometimes, to prevent ethnic cleansing, and have gone to war to reverse territorial conquest. The restraint showed by other nations has not been a sign of human progress, the strengthening of international institutions, or the triumph of the rule of law. It has been a response to a global configuration of power that, until recently, has made restraint seem the safer course.

This is a very simple and self-serving explanation, but does it make much sense? The U.S. has led international coalitions to repel international aggression a few times in the period Kagan mentions. That didn’t prevent cross-border aggression during the intervening decades, and it hasn’t stopped it from happening since, so why would that be the cause of an overall decline in interstate warfare? The U.S. has intervened on ostensibly humanitarian grounds a little more often than that, but most of these have taken place in the last twenty years and don’t explain changes in behavior before that. If there has been a significant decline in interstate warfare and a decline in violence within states since WWII, this cannot simply be a function of U.S. and allied power. If it is so difficult to influence other nations, as Kagan admits elsewhere in the essay, how is it that sporadic, inconsistent applications of power by Western governments produced such a significant change in the behavior of almost all other states? Furthermore, how did the U.S. and its allies produce this change when they were engaged in their own unnecessary and illegal wars during the same period? Kagan seems to think that other nations have exercised restraint only because they are afraid of what the U.S. might to do them if they did not, but that seems wrong. It certainly doesn’t give other nations any credit for being able to understand the value of peace on their own.

That doesn’t mean that the U.S. and its allies have had nothing to do with the change, but it suggests that most nations are genuinely more inclined to not resort to the use of force than they used to be. Something other than U.S. and allied power would seem to be mostly responsible for this change, which suggests that U.S. hegemony isn’t necessary to keep that trend going. I don’t think that anything in human nature has changed, but it isn’t so far-fetched to think that since the end of the world’s most devastating conflict most nations have come to recognize on their own that resorting to the use of force is foolish and undesirable. Obviously, nations can forget what their ancestors have learned through bitter experience, and so there is nothing permanent about this condition. Nonetheless, we shouldn’t presume that today’s relatively peaceful world depends so heavily on the exercise of American power, nor should we accept the assertion that the world will become dramatically less peaceful without having the U.S. constantly policing it.