The ability—nay the eagerness—to turn conquered peoples into Romans was the key: Rome after 340-338 replaced ethnicity and geographical location as the basis of membership in the polity with a ladder of legal status-groups not tied to either…. non-Roman allies (the socii), halfway citizens (the cives sine suffagio), full citizens (cives)…. the Romans were… generous in allowing non-Roman individuals and even (very occasionally) whole non-Roman polities to climb up this status hierarchy, Rome gained an enhanced capacity to win loyalty…. The capacity for inclusion and integration…. The rewards Rome was culturally and politically capable of dispensing… local peace… [and] the inclusion of local elites in the Roman state were much greater than any state in Greece could ever offer…. It was the system of inclusion, which was ultimately a diplomatic and political skill… that made Rome potentially the most powerful state in the ancient Mediterranean…
This is fundamentally why the loss of the Socii in the Social War to the Optimates was so damaging. Although the next administration ended up giving the Socii the citizenship they'd demanded, the aristocratic opposition to expansion of citizenship started feeling like they could get away with anything -- following which the Republic degenerated in a succession of military coups, starting with Sulla's and capped by Julius Caesar's. Having lost its fundamental democratic basis, Roman power proceeded to collapse over the next 100 years in internal wars.
This is fundamentally why the loss of the Socii in the Social War to the Optimates was so damaging. Although the next administration ended up giving the Socii the citizenship they'd demanded, the aristocratic opposition to expansion of citizenship started feeling like they could get away with anything -- following which the Republic degenerated in a succession of military coups, starting with Sulla's and capped by Julius Caesar's. Having lost its fundamental democratic basis, Roman power proceeded to collapse over the next 100 years in internal wars.