This paper considers the productivity impact on the US economy of the period of war mobilization and demobilization lasting from 1941 to 1948. Optimists have pointed to learning by doing in military production and spin-offs from military R & D as the basis for asserting a substantial positive effect of military conflict on potential output. Productivity data for the private non-farm economy are not consistent with this view, as they show slower total factor productivity (TFP) growth between 1941 and 1948 than before or after. The paper argues for adopting a less rosy perspective on the supply side effects of the war.