Military of ancient Rome in the context of "Celeres"

The Roman army (Latin: exercitus Romanus) served ancient Rome and the Roman people, enduring through the Roman Kingdom (753–509 BC), the Roman Republic (509–27 BC), and the Roman Empire (27 BC – AD 476/1453), including the Western Roman Empire (collapsed AD 476/480) and the Eastern Roman Empire (collapsed AD 1453). It is thus a term that broadly spans approximately 2,206 years, during which the force underwent numerous permutations in size, composition, organization, equipment and tactics, while conserving a core of lasting traditions.

The Roman imperial cult (Latin: cultus imperatorius) identified emperors and some members of their families with the divinely sanctioned authority (auctoritas) of the Roman State. Its framework was based on Roman and Greek precedents, and was formulated during the early Principate of Augustus. It was rapidly established throughout the Empire and its provinces, with marked local variations in its reception and expression.

Augustus's reforms transformed Rome's Republican system of government to a de facto monarchy, couched in traditional Roman practices and Republican values. The princeps (emperor) was expected to balance the interests of the Roman military, Senate and people, and to maintain peace, security and prosperity throughout an ethnically diverse empire. The official offer of cultus to a living emperor acknowledged his office and rule as divinely approved and constitutional: his Principate should therefore demonstrate pious respect for traditional Republican deities and mores.

The structural history of the Roman military concerns the major transformations in the organization and constitution of ancient Rome's armed forces, "the most effective and long-lived military institution known to history." At the highest level of structure, the forces were split into the Roman army and the Roman navy, although these two branches were less distinct than in many modern national defense forces. Within the top levels of both army and navy, structural changes occurred as a result of both positive military reform and organic structural evolution. These changes can be divided into four distinct phases.

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, the 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.

The Roman army battled first against its tribal neighbours and Etruscan towns within Italy, and later came to dominate the Mediterranean and at its height the provinces of Britannia and Asia Minor. As with most ancient civilizations, Rome's military served the triple purpose of securing its borders, exploiting peripheral areas through measures such as imposing tribute on conquered peoples, and maintaining internal order. From the outset, Rome's military typified this pattern, and the majority of Rome's campaigns were characterised by one of two types. The first is the territorial expansionist campaign, normally begun as a counter-offensive, in which each victory brought subjugation of large areas of territory and allowed Rome to grow from a small town to a population of 55 million in the early empire when expansion was halted. The second is the civil war, which plagued Rome from its foundation to its eventual demise.

The scorpio or scorpion was a type of Roman torsion siege engine and field artillery piece. It was described in detail by the early-imperial Roman architect and engineer Vitruvius in the 1st century BC and by the 4th century AD officer and historian Ammianus Marcellinus.