Goldsworthy is perhaps the best living writer on Rome. This brilliant book follows the development of the Roman Army from its mythic founding to the collapse of the West in the 5C BCE, over about 1,200 years. The writing is dense and to the point, skipping narrative treatment for hard-nosed analysis. He seeks to answer the questions of what made the Roman Army unique, how it adapted to different circumstances, and what interplay of its relation to the political institutions, both as protector and source of change, and finally what its legacy is.
For the first few centuries, the Roman Army was essentially a citizens' militia, drawn from the landed classes (largely farmers for the infantry) and commanded by the patrician aristocracy in the Senate. They were not particularly well organized, rushing into battle to establish heroic honor, much like the soldiers of Alexander. But there were differences that survived for a millenium: Rome always fought to win instead of seeking tactical advantage and, rather than subjugating conquered peoples, they attempted to coop them, first by leaving their elites in charge and then by employing them as foot soldiers.
As Rome consolidated her hold over central and southern Italy, her armies also began to organize themselves into larger units, particularly after the city was occupied by the Celts. In the first excursion outside of Italy (to Sicily to fight Carthage), Rome developed a navy, which they rebuilt several times with relentless determination. In the following war with Carthage, under attack from Hannibal, the Roman legions developed a new flexibility and control, enabling them to fight also the Hellenistic Greeks; in both of these conflicts, the Romans would not back down in the face of reversals and would continue to fight even when it had advantage, refusing deals that would maintain the status quo at lower cost. Eventually, they were beaten and incorporated or annihilated in the case of Carthage, a culture that seemed to be reborn at each opportunity.
Rome emerged as the greatest power in the mediterranean, supremely confident and striking fear into its potential enemies. It was the Roman Republic's golden age, a period of rapid expansion. Goldsworthy goes into great detail about the technical organization, which I will not attempt to recapitulate (e.g. you can learn how many men a centurion commanded and what his job was, etc.). Suffice to say, they developed a formation with flexible lines; younger soldiers began in front, followed by middling ones, and finally, older ones who were experienced and held in reserve. They could form and reform, allow openings for purposes of maneuver and entrapment. And flanked by cavalries staffed by equestrians, those wealthy enough to equip themselves. The Army had great supply lines, supported by huge investment from the society, and a great flexibility to adapt to changing circumstances.
Nonetheless, Rome was run very much as a city state, with minimal bureaucracy and a kind of amateurism in the ever changing chief executives (consuls), who served only a year and had to lead their campaigns within that time frame; this limited their vision and staying power, leading often to the mistreatment of their colonial charges. This amateurism led to the social wars, the last great rebellion on the Italian peninsula. To meet it and the many challenges in disparate parts of the growing empire (Celts/Germans and Jugurtha in N Africa), a series of great generals (in particular Caius Marius) began to professionalize the army. Though command remained largely in the hands of Senators, poor men replaced the yeomen farmers who normally fought at the expense of their farm duties. This had terrible ramifications. First, if this is not propaganda, it lessened the civic obligation and consciousness of the troops, who had less at stake than land-owning farmers in the nation and did not view the republic as religiously inviolate. Second, with the refusal for political reasons to take care of retired veterans, they gave their loyalty exclusively to their generals. Third, the army was becoming autonomous, a power center of its own, and in the future they would become the kingmakers. The legitimacy of the Republic gave way to fealty to generals, and terrible civil wars ensued, pitting Marius against Sulla and Julius Caesar against Pompey, to name the biggest ones.
Once the republic was dead and Marc Antony finally defeated, Augustus further professionalized the army, moving it into many roles of administration and policing, ensconcing it even further into civil society. Here, his pretorian guard - the elite military protectors of the Emperor in Rome itself - moved into a new position of power which was to last for over 300 years. This represented a fundamental shakeup, eliminating the old aristocratic families from their traditional "rights" as commanders and colonials potentates; while gifted with privileged access, they would have to earn their places now by skill rather than politics. Once Augustus established the Imperium, which posited that Rome has ended its expansion policy (later to be revisited, of course), Rome became an essentially defensive power, preserving rather than expanding without limit. It was no longer a question of seizing glory as it was to provide security, which had the obvious consequences: a new (and realistic) conservatism, a rolling back of ambitions, and a defensive posture. Fortifications began to be built, such as Hadrian's wall, and the army was reorganized for these contingencies, perhaps with over confidence.
As generals became more powerful in their remote outposts, they became direct threats to the authority of the emperor. This created a paradox: the emperor needed good soldiers, but couldn't trust them. After the golden age of the imperial empire with Marcus Aurelius, a terrible period of instability began, with changes in leadership every few years. Generals were elevated by their troops to emperor, essentially usurpers by proclamation, initiating innumerable civil wars that depleted manpower and resources, while the borders became less secure and adversaries such as the Germans learned to adopt many of the old tactics of the Roman army.
It was only with Diocletian that Rome recovered. Interestingly, as an Albanian, he was one of the first commanders who was not of the old Roman aristocracy, but a new man elevated purely by his competence. The army, at last, was becoming wholly professional. While it was feared, it also enabled Diocletian to reorganize the entire society around the army, with a more accurate census and even distributed taxes instead of local levies in times of need; he also created co-emperors he trusted, allowing them to act competently and without fear; he eliminated the Praetorian Guard, removing a crucial political destabilizing force. He also remolded the bureaucracy to service the army. Diocletian moved the border outposts farther in, so that they could strike rapidly wherever there was trouble and lessened the necessity of protecting the impossibly long frontier. Except for the Parthians and Persians, military engagement against barbarians became an affair of small raiding parties, which the Roman army learned to counter quickly. Unfortunately, this system proved unsustainable, and civil wars increased at the moment of population decline and increasing use of barbarians and even mercenaries. Multiple poles of independent power centers established themselves in the Roman west, leading to a breakdown in the military system through lack of investment and loss of faith in the invincibility to the army. When it collapsed, it did so very quickly. (This does not explain the fall of Rome, that would take another book.)
This makes for fascinating reading for the history buff. There were many points of Roman history that I saw clearly for the first time, many points of clarification, and even some questioning of accepted wisdom. It is the performance of a true master of popularization that is also accurate as a scholarly work - a masterpiece. Goldsworthy writes the most elegantly lucid prose. I have no real criticisms of the book. Recommended with enthusiasm.