For more than twelve centuries, from its legendary founding in 753 B. C. to its renowned fall in A. D. 476, ancient Rome existed as an independent state. During this long interval, the Roman people and their government, society, and culture steadily evolved, changing in response to an everchanging world, as well as to a constant influx of new ideas and customs borrowed from other peoples. All the while, the Roman realm relentlessly expanded and contracted. At first consisting of a tiny city-state in west-central Italy, Rome grew into a vast empire encompassing the whole Mediterranean world; eventually, this process reversed itself and that empire shrank back into a small Italian state of marginal power and influence.

Simultaneously, responding to the needs of the Roman state, the Roman army underwent an evolution of its own. It...