James Kurth raises the interesting possibility that the nations with below-replacement birth rates -- not only the industrialized nations, but also China -- will not want to risk their precious only children as cannon fodder in major land wars. Such a policy would, obviously, have its up side if it reduced the chance of war. On the other hand, there are times when a credible military, and the will to use it, are necessary for defense.
It is certainly true that the total birthrates of Western nations are below replacement, often well below. But these birthrates have not been spread evenly to produce a majority of one-child families. Rather, we have a rising number of women with no children, averaged in with the two or three child families of the middle class, and the three or four child families of the poor and immigrants.
Kurth rightly says that the "liberal professionals and professional liberals" are particularly unlikely to have large families. However, they are also particularly unlikely to send their children to war, no matter how many they have. The war-making class and the war-fighting class are as separated as they ever have been, thanks to the all-volunteer force. Short of a large draft, things will stay that way.
Demographic contraction will have a significant effect on how big a force we could muster in a large war. But I don't think the trend toward few children in the educated classes will translate into any greater reluctance to make war at all. Chelsea Clinton and the Bush twins were the precious result of their parents' only birth, but it did not make their commander-in-chief fathers any less likely to commit the country to war. Those kids weren't going to serve, anyway.