Concerns over free riding in NATO are widespread. An intuitive approach to analyzing free riding is treating it as a systematic pattern of spatial interdependence between the allies: how does a NATO member's defense spending react to changes in other allies' military expenditures? While recent work has found statistically significant free riding (negative spatial interdependence in the outcomes), it suffers from important limitations. First, this research does not adequately account for temporal dependence. Second, it does not quantify the effect of interest. Accounting directly for temporal dependence provides a meaningfully distinct perspective on the within-alliance dynamics, demonstrating that the spatiotemporal effect of free riding is, in fact, more substantial than its short-run effect, challenging inferences of static spatial models. We discuss the relevant practical and theoretical implications.

Suppose it is foreseeable that you will soon encounter a drowning child, whom you will only be able to rescue if you learn to swim. In this scenario we might think that you have a “prospective duty” to take swimming lessons given that this will be necessary to perform the future rescue. Cécile Fabre argues that, by parity of reasoning, states have a prospective duty to build and maintain military establishments. My argument in this essay pulls in the opposite direction. First, I emphasize that learning to swim is only a prospective duty under very specific circumstances. Normally there is no such duty; hence, we do not normally think that people deserve moral censure for choosing to forego swimming lessons. I then argue that, similarly, while a prospective duty to build a military can arise under some conceivable circumstances, these are not the circumstances that most states today find themselves in. I then suggest a more fitting domestic analogy to guide our thinking about this issue: Maintaining a standing army is less like learning to swim and more like keeping an assault weapon in the home “just in case.” This analogy supports a defeasible presumption against militarization.