The military of ancient Rome was one of largest pre-modern professional standing armies that ever existed. At its height, protecting over 7,000 kilometers of border and consisting of over 400,000 legionaries and auxiliaries, the army was the most important institution in the Roman world. According to Titus Livius, one of the more illustrious historians of Rome over the centuries, the military was a key element in the rise of Rome over "above seven hundred years" from a small settlement in Latium to the capital of an empire governing a wide region around the shores of the Mediterranean, or, as the Romans themselves said, mare nostrum, "our sea". Livy asserts:... if any people ought to be allowed to consecrate their origins and refer them to a divine source, so great is the military glory of the Roman People that when they profess that their Father and the Father of their Founder was none other than Mars, the nations of the earth may well submit to this also with as good a grace as they submit to Rome's dominion.

From its origin as a city-state on the peninsula of Italy in the 8th century BC, to its rise as an empire covering much of Southern Europe, Western Europe, Near East and North Africa to its fall in the 5th century AD, the political history of Ancient Rome was closely entwined with its military history. The core of the campaign history of the Roman military is an aggregate of different accounts of the Roman military's land battles, from its initial defense against and subsequent conquest of the city's hilltop neighbors on the Italian peninsula, to the ultimate struggle of the Western Roman Empire for its existence against invading Huns, Vandals and Germanic tribes. These accounts were written by various authors throughout and after the history of the Empire. Following the First Punic War, naval battles were less significant than land battles to the military history of Rome due to its encompassment of lands of the periphery and its unchallenged dominance of the Mediterranean Sea.