The Roman Army was disciplined, particularly compared to many of the peoples it faced. It was not unbeatable in battle; it consisted of ordinary humans, and their track-record is as chequered as one would expect of an institution that existed for over a thousand years and saw continuous evolution, adaptation and change. Its fighting prowess is somewhat exaggerated in the popular imagination, as well as that of some professional historians (Arther Ferrill comes to mind).

That said, the Roman army was incredibly effective for much of its history, and victories in battle were extremely common. Whatever institutional, demographic and cultural factors may have contributed to the success of their empire, it's hard to imagine Roman rule spreading over so much of their known world without the simple fact that they won far more battles than they lost.

Even more importantly: Rome may have lost many a battle, but it lost very few wars. In the days of the Republic, the Romans were notoriously reluctant to negotiate peace unless it happened on their terms, and their usual response to defeat in battle was to mobilise more resources, raise new armies, and try again until victory was achieved. Every one of those famous Roman defeats you may have heard of, be it the defeats against Hannibal at the river Trebia, lake Trasimene and Cannae, against the Cimbri and Teutones at Arausio, against the Parthians at Carrhae, or against the Germanic tribes in the Teutoburger Wald, were followed by Roman victories: the battles of the Metaurus, of Ilipa, and Zama against the Carthaginians, the battles of Aquae Sextae and Vercellae against the Cimbri and Teutones, the battles of the Cilician Gates, the Amanus Pass and Cyrrhestica against the Parthians, the battle of the Weser River against Arminius' Germans.

Of course, not every Roman war started out with defeats. Many of Rome's biggest conquests happened with nary a hitch. The conquest of Greece, Macedon, Syria, Asia Minor and much of the rest of the middle east, as well as Caesar's conquest of Gaul, all involved virtually unbroken strings of Roman victories. The battles of Cynoscephalae, Magnesia, Pydna, Tigranocerta, Bibracte and Alesia are some of the more famous Roman victories in those conflicts.

When examining the Roman army's track record, it's important to remember that the army of the 4th century B.C. was not the same as that of the 2nd, which in turn was different from that of the 1st, which was different again from the army of the 1st century A.D., which had changed even further by the 4th century A.D., etc, etc. The Roman army started out as a part-time militia force, consisting of men who would spend most of their lives on their farms, until they were called into the army for a few campaigns. It was only by the 1st century A.D. that the army started to evolve into the fully professional force that we know from Hollywood films, a process that was more or less complete at the start of the Common Era. It's probably not surprising that the militia army had a more variable track record than the imperial one, though in fact it was the militia army of the middle republic that defeated the most powerful states on the Mediterranean, such as the Carthaginians, Macedonians and Seleucids.

By the time of the empire, though, there simply weren't any organised states with professional armies left, except Rome itself, and consequently the Roman army won the vast majority of its battles. Even when the enemy had the advantage of numbers, (which can be tricky to determine: our sources aren't exactly without bias) the Romans had superior logistics, superior equipment, and training on a scale that your typical Germanic or Gothic force simply could not match. This remained true even in the empire's later days.