Most small farmers were not tied to a larger land owner or tenant for the Republic and most of the Empire (as I have alluded to in another post about the latifundia being much smaller then traditionally thought). The change came in late antiquity when the central authority and government began to deteriorate, the local authorities began to wall cities due to barbarian invasions and local elites would provide protection from their walled villa's creating a pseudo-feudalism. Furthermore the military was structured differently in Ancient Rome than Feudal Europe. The Emperor would have absolute authority over the army although often appointing generals unless leading personally. This army was also professional and stationed on the frontier provinces serving for a period of time; up to 20 years in "military provinces" under the direct control of the Emporer usually governed by a legate versus a "senatorial province" governed by a senator. Feudal Europe operated differently with the king having looser control over the lesser nobility with a levy being called to serve each season of war and protected by laws to return to thier farms.