Roman legions

The Roman army (Latin: exercitus Romanus) is a term that can in general be applied to the terrestrial armed forces deployed by the Romans throughout the duration of Ancient Rome, from the Roman Kingdom (to c. 500 BC) to the Roman Republic (500–31 BC) and the Roman Empire (31 BC – 395/476 AD), and its successor the East Roman or Byzantine Empire. 
The military of ancient Rome, according to Titus Livius, one of the more illustrious historians of Rome over the centuries, was a key element in the rise of Rome over “above seven hundred years”[1] 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.”
The Early Roman army of the Roman Kingdom and of the early Republic (to c. 300 BC): during this period, when warfare chiefly consisted of small-scale plundering raids, it has been suggested that the Roman Army followed Etruscan or Greek models of organisation and equipment. The early Roman army was based on an annual levy.
The infantry ranks were filled with the lower classes while the cavalry (equites or celeres) were left to the patricians, because the wealthier could afford horses. Moreover, the commanding authority during the regal period was the high king.