Intelligence operations face the challenging task of predicting the shape of future wars. This task is hindered by their limited ability to warn of peacetime foreign military innovation. Using formerly classified sources in particular, the reports of military attaches and other diplomat-officers Thomas G. Mahnken sheds light on the shadowy world of U.S. intelligence-gathering, tracing how America learned of military developments in Japan, Germany, and Great Britain in the period between the two world wars.

The interwar period witnessed both a considerable shift in the balance of...