The Military Affairs of Ancient Rome
When I said a while ago that the organization of the Roman Army was such as to be thought worthy of study even in our day, some might have thought I was exaggerating; well, here's an online book that ought to dispel any scepticism on that score once and for all. After reading this, the reason why the Romans came to rule the whole of the Mediterranean will be as clear as can be - the real question isn't "why were the Romans so much better than everyone else at fighting?", but just how much further they could have gotten were they not limited in their expansionism by logistical considerations.
Another interesting question that suggests itself is why an empire with such a thoroughly trained, flexible and highly professional army should ever have fallen to hordes of undisciplined barbarians. From what I've gleaned from my readings thus far, at least part of the explanation for this lies in the fact that those "undisciplined barbarians" weren't quite as undisciplined as all that to begin with; after centuries of serving either alongside Roman legions as auxiliaries or actually serving within said legions, it was only natural that the Germanic tribesmen should have picked up a thing or two about sophisticated military campaigning. It must not be forgotten that Alaric, who sacked Rome in 410 AD, had been serving as a leader of Roman foederati* since the year 394, and was therefore thoroughly familiar with what he was likely to come up against; even so, the Romans were still more than a match for him as late as 402 AD, when his first invasion of Italy was checked by a commander named Stilicho. In fact, Alaric might probably never have succeeded in capturing Rome at all, were it not for the bitterness of the relations between the courts of the eastern and western halves of the Roman empire, as well as the intrigues that were taking place within the court of Flavian Augustus Honorius.
PS: Here's more information on the subject, this time from a source that ought to know about such things. I'll quote a key passage that really is astonishing considered in light of the interval under discussion:
The training regimen of the Roman soldier was necessitated in large degree by the use of sophisticated, open formations by the infantry. Denied the protection of the closely packed phalanx, the Roman soldier lived or died by his skill with the sword. The need to fight as an individual and to move over a designated area, selecting targets of opportunity while remaining still part of his larger unit, required courage, discipline, and skill with the sword and scutum ,[note] operating in concert. Roman tactics required the soldier to be able to respond instantly to commands to change the shape of his formation. In 105 B.C., the Roman army adopted the training methods heretofore used by professional athletes in the gladiatorial schools. For the most part the legions trained their own soldiers. Special training grounds, some in Scotland, were available to bring the army to proficiency. It was common practice for a legion being readied for deployment to spend the previous weeks in long field training drills, some of which required that they build three field camps a day. The result was a thoroughly professional army whose level of training was the best in the world.Note that the emphases in sentences above were all in the original document. The final sentence is particularly telling: the Europeans of the post-fall era managed to forget most of what they learned even about fighting from the Romans, despite the fact that warfare took up so much of their time and energy. It's hard to think of better evidence that yes, the Dark Ages really did mark a steep decline in knowledge and organizational sophistication.
No army in the West equaled the level of training of the Roman army until at least the 17th century.
*"Allies" - the Roman name for foreign auxiliaries.