War and the extent of mass mobilization for war has a significant impact on a wide variety of economic and political development outcomes. In this paper, we investigate to what extent technological change has influenced the choice by governments to field mass armies. Focusing on a sample of thirteen great powers between 1600 and 2000 we argue that changes in transport and communications technology were the single most important factor that ushered in the era of the mass army and subsequently led to its demise. During the nineteenth century the development of the railroad made it possible for the first time to mobilize and feed armies numbering in the millions. During the late twentieth century further advances in transport and communications technology made it possible to deliver explosive force from a distance and with precision, making mass armies less desirable. We find strong support for our technological interpretation using a new data set that measures army size, population mobilization, and methods of recruitment from the beginning of the seventeenth century. In so doing we also consider several other plausible determinants of military mobilization. Contrary to what is often suggested by scholars, we find little evidence that the French Revolution and the invention of the concept of «the nation in arms» was associated with a substantial increase in levels of mobilization across nations. Even for the French case alone, the magnitude of what is sometimes referred to as the «Napoleonic watershed» was smaller than what is often believed.