Short answers, pulled from the story.
The Roman army, known as the Exercitus Romanus, endured for approximately 2,206 years, from the first universal levy organized around 550 BC under king Servius Tullius to the fall of Constantinople on the 29th of May 1453 and the conquest of the last Byzantine remnants by 1461.
Around 300 BC, the phalanx was replaced by the manipular system. Armies were deployed in three lines of small units called maniples, each containing 120 men and arranged in a staggered, chessboard pattern that gave commanders far greater tactical flexibility on broken or variable terrain.
Augustus transformed the legions from a mixed conscript-and-volunteer corps into an all-volunteer force serving a standard 25-year term, reserving conscription for emergencies. He also created the auxilia, a corps of non-citizen soldiers who provided cavalry, archers, and specialist infantry, and who received Roman citizenship on completing their minimum service.
The Roman imperial army reached approximately 450,000 men by 211 AD, organized into 33 legions and around 400 auxiliary regiments, with auxiliaries outnumbering legionaries substantially by that point. At the end of Augustus's reign in 14 AD, the force numbered roughly 250,000 men split equally between the two corps.
The Social War of 91 to 88 BC broke out because Rome's Italian allies, the socii, had fought Rome's wars for generations but were excluded from public land redistribution as non-citizens. The settlement granted Roman citizenship to all Italians, dissolved the separate allied formations, and absorbed the socii directly into the legions, ending the army's long-standing dual structure.
The Roman army's capacity for recovery rested on its structural ability to expand the recruitment base through citizenship extension and allied incorporation. As the historian Polybius observed, Roman defeats triggered renewed mobilization rather than political collapse, a systematic advantage that Sparta's shrinking Spartiate class and Macedon's royal-dependent agrarian base could not match.