When the Roman soldier ceased to be the determined, enthusiastic, effective fighter he had been for many centuries, Rome's days were numbered. Indeed, the decline of the Roman military and the often-discussed "decline and fall of the western Roman Empire" are intimately related. Over time, poor pay and training, loss of prestige, high desertion rates, increasing recruitment of less disciplined non-Roman Germans, and other factors greatly reduced the effectiveness of Rome's armed forces. Deprived of the chief tool it needed to keep its territories from falling to the northern tribes, the central government could not stop the western realm from steadily shrinking. And eventually, the last remnants of that realm fell under the control of an army that called itself Roman but was in fact made up of and commanded by Germans. This was part of the ongoing...