That's a big question.

Still, I'll give it a shot, though I'm not sure I can avoid a bit of a ramble.

We can immediately start with a few obvious facts:

• The Romans conquered the entirety of the Mediterranean world that birthed their state, and much of what lay beyond. They did so mostly through the direct projection of military power, though their use of diplomacy and "soft power" should probably receive more attention than it gets in the popular imagination.
• War in this period was mostly decided through pitched battles, which the Roman legions fought and won innumerable times over the course of many centuries, though they suffered many dramatic and sometimes crushing defeats as well.
• They waged wars against all manner of peoples, fielding all kinds of armies, from highly organised professional or mercenary armies, to tribal levies led by warrior aristocracies, to all-cavalry forces of steppe nomads, to guerilla warriors fighting through hit-and-run tactics, to their own allies using their own military system against them. They defeated and conquered all of these foes.

On the face of it, it is difficult to conceive of the Roman empire coming about the way it did without the Roman army being very good at what it did, and from the time of the Republic up to and including the Principate, what the Roman army did was fighting and winning decisive pitched battles.

At the same time, it is easy to get carried away by the boy's own mystique of glory. The Romans lost a lot of battles, even if they won more. The Roman legions certainly have been mythologised, both in the popular imagination and by prominent military historians. Arther Ferrill goes so far as to say:

Against barbarian armies Roman forces were invincible, barring stupid mistakes of generalship. In fact, because of their training, Roman armies had at least a 4-to-l superiority, that is an army of 20,000 Romans could defeat an army of 80,000 barbarians.^1

I don't think such claims are at all justifiable. (Although here I should admit that I am not a fan of Ferrill's work in general.) Sure, there are instances where Roman armies beat such odds. (At least according to Roman sources, and possibly sometimes in reality, though the arguments over ancient numbers of soldiers and casualties are endless.) But a-symmetrical victories and defeats (and hugely inflated numbers) are a hallmark of pre-modern combat in general, due to the huge impact of morale on the outcome and the relative lack of killing power of pointy sticks, and Romans lost battles against the odds as well as won them.

Most modern military history (and much older work as well) stresses the organisational and logistical strengths of the Roman empire. During the Republic, the Romans had an incredible ability to absorb and recover from losses, which is a testament to a rather unique political and societal will to fight and pursue victory. Many (i.e. Goldsworthy) would credit this tenacity (or as some might say, stubbornnes) more than their sheer prowess in battle, when trying to explain the Romans' success.

During the Principate, against most of those barbarian foes Ferrill mentions, the Roman could bring more force to bear than they could possibly hope to deal with. Not just in terms of numbers, but also in the ability to keep an army in the field for year after year, to deploy and supply forces that would strike deep into enemy territory, and to inflict painful losses on the enemy homelands whilst never letting invaders pass beyond their own peripheral territories. These factors certainly must not be underestimated. Even when considering the most advanced of Rome's neighbours, the Parthians and later the Sassanid Persians, we have to accept that Roman armies sacked their capitals time and time again, whilst the eastern empires never managed to seriously threaten Rome's existence until after the fall of the West and the weakening of Rome's power that involved. Once the Roman empire was established, the simple fact of the matter was that nobody was big enough to seriously threaten it.

On the other hand, Rome worked very hard to maintain a reputation of invincibility, and stubbornly refused to accept peace without first winning victory in battle. (or without first doing something they could spin as such.) It is unlikely that they could have maintained their empire as long as they did without the ability to keep winning many more battles than they lost.

So, we should try to nail down those questions of "tactical sophistications and discipline" in detail. Which is considerably harder than generalities such as I offered above. To attempt to answer that question, we have to rely on:

• Military manuals — These offer the most clear evidence on how the Romans thought battles ought to be conducted. However, they describe theory and not necessarily practice, and in any rate the really detailed ones tend to be from the Byzantine period, or the highly problematic Vegetius.
• Narrative sources and histories — this is our best bet when considering big-picture stuff like this. On the plus side, we have quite a few, they have been exhaustively studied, and we can fairly easily compare and contrast Greek, Hellenistic and Roman (or Roman-era Greek) sources. On the downside, most of them tend to be remarkably vague on the actual mechanics of battle, which has led to all kinds of long-running academic debates on what these guys with pointy sticks actually did when they were supposed to kill one another. In addition, most of our best sources describe Republican battles. The sources become much vaguer and more limited when we enter the Imperial period.
• Other documentary sources — Papyri from Egypt and the odd lucky find elsewhere, such as Vindolanda, offer a wealth of insight in how the Roman army was organised and conducted its day-to-day affairs. Unfortunately, they tell little about tactics in the field. After all, the army only spent a tiny percentage of its time actually fighting. Additionally, these sources only apply to the Imperial period, and even then are (as mentioned) limited geographically. The Republican army, the one that actually won the Romans their empire, left few records.
• Archaeological material — few things are as exciting (if you're a Roman military history nerd) as news that another ancient battlefield has been dug up. Unfortunately, such are extremely few and far between, and even then the passage of time is such that the resulting finds will offer scattered and not-necessarily-representative details rather than anything that easily lends itself to constructing a big picture.

Exhausitively discussing all of those is... well, not on my agenda for today. If I even could. Fortunately, rather than writing a dissertation, I can approach the question from a different angle: Tactically sophisticated, disciplined and skilled compared to whom, or what?

Leaving aside those who thought it fruitful to compare Alexander's phalanx to the army of Napoleon,^2 it only really makes sense to compare the Roman legions to their contemporaries, and perhaps with slightly greater difficulty to their immediate successors and predecessors. And this makes our task much, much easier.

If we take the things the Roman legions are described as doing on the battlefield in our sources, and compare these to what their contemporaries and enemies are described as doing, we immediately note that the Romans are capable of things that most of their enemies are not. A Gallic or Germanic force can form a shieldwall or phalanx, it can attack aggressively or defend tenaciously, and it is more than capable of staging an ambush. But such armies are never described as performing any kind of sophisticated mid-battle manoeuvre, such as Scipio redeploying his army mid-way through Zama, Marius arranging for a detachment to launch a surprise rear-attack at Aquae Sextiae, or Caesar using his concealed fourth line to rout the Pompeian cavalry at Pharsalus.

Conversely, when analysing the battles of Hannibal or Alexander the Great, we do see such feats performed. Additionally, it should be remembered that most of the battles we're told about involve no such fancy manoeuvring, either on the part of the Romans or their enemies, and that attempts to do so could backfire. (Before the battle of Bibracte, Caesar attempts such an encirclement by sending Labienus ahead with two legions, but later his scouts mistake the Roman detachment for the enemy. Disaster might well have ensued.)

^* ^1 ^Arther ^Ferrill, ^Roman ^Imperial ^Grand ^Strategy ^p.21

^* ^2 ^Arther ^Ferrill ^again. ^The ^origins ^of ^War, ^to ^be ^precise.