We investigate how technology has influenced the size of armies. During the nineteenth century, the development of the railroad made it possible to field and support mass armies, significantly increasing the observed size of military forces. During the late twentieth century, further advances in technology made it possible to deliver explosive force from a distance and with precision, making mass armies less desirable. We find support for our technological account using a new data set covering thirteen great powers between 1600 and 2000. We find little evidence that the French Revolution was a watershed in terms of levels of mobilization.