Hans Delbruck (November 11, 1848 - July 14, 1929) was a German historian. Delbruck was one of the first modern military historians, basing his method of research on the critical examination of ancient sources, the use of auxiliary disciplines, like demography and economics, to complete the analysis and the comparison between different epochs to trace the evolution of military institutions.

Moral decadence did not contribute to the fall of the Roman Empire, but political instability that was most strongly reflected in a weakened army did, writes Hans Delbruck in volume 2 of History of the Art of War. The rise of sectionalism and the constant overthrowing of emperors left the empire without a secure superior authority demanding the loyalty of the legions and without the money to support them. Delbruck’s celebrated work traces the collapse of the Roman military system and its replacement with barbarian mercenaries by the fourth century, following the invasions by the Germanic peoples.

The Barbarian Invasions opens with a discussion of the military organization of the early German tribes, compares it with that of the early Roman legions, and goes on to show how tactics and strategies changed after entire peoples such as the Goths, Vandals, and Burgundians migrated to Roman soil and formed an uneasy alliance with the Romans. It ends with an examination of the decline of the Germanic-Romanic military system in the Middle Ages and the rise of the feudal system.

This Bison Book edition marks the first appearance in paperback of the English translation of volume 2 by Walter J. Renfroe Jr.