Japan’s strategy-resource mismatch creates the potential for all sorts of geostrategic problems. Without manpower the Japanese defense forces will necessarily rely on advanced technologies. But tech doesn’t come cheap. And the very expense of high-tech military systems makes it more difficult to justify placing them in harm’s way.

In a sense, Japan could fall into the same trap that Western Europe already faces: the inability to formulate proportional military responses. Developed countries in demographic decline may have nuclear weapons, but for the most part, they lack basic military manpower. So when faced with a security threat or provocation, they have only two options: passivity and overreaction. Neither is conducive to peace.

There are other, tertiary effects of fertility collapse that contribute to instability. For instance, in some parts of the Third World, fertility decline has been so rapid that age structures are being scrambled by generational “echo booms.” In a chapter on the geopolitical implications of aging, Phillip Longman notes that in Iran between 2005 and 2020, the number of people aged 15-24 will shrink by 34 percent. That’s a startling shift. But then, because of echo effects, the 15-24 cohort will grow by 34 percent between 2020 and 2035. Even a stable, well-ordered society would have trouble coping with such wild demographic dislocations.