I've gotta say, the first entry over at the new Becker-Posner blog, along with the accompanying citation, is just plain dumb.
In a nutshell: Posner says war is justified if the expected costs are less than the expected gains. He then illustrates this with a nice example: if the costs of inaction are 35 and the cost of war is 5, then war is justified.
No, this is not a joke (as far as I can tell - though Kieran Healy thinks differently).
The accompaning paper has some simple models and calculations of thresholds for when its OK to attack.
Obviously, all of this is silly and useless because in the real world, one can never know any of the parameters involved: the probability of an enemy attack, the costs of an attack, the number of casualties, civilian or military, the duration of the war, the economic effect, and so on. It would be nice if we could say, in any particular instance, that the cost is 35 but we will never be able to do so, not even approximately.
If this is what passes for scholarship in the social sciences, I'm pretty happy with the career choices I made.