Following the expulsion of Tarquin, Brutus, whom the comitia elected one of the first consuls,[i] commanded the cavalry in the Battle of Silva Arsia, where he fell, BC 509.
[4][5] In the early years of the Republic, no attempt was made to reconstitute the office of Tribune of the Celeres; the supreme military authority was vested in the consuls.
But in the ninth year of the Republic, war appeared imminent with both the Latin League, led by the exiled king's son-in-law, Octavius Mamilius, and the Sabines, with whom the Romans had fought in both 505 and 503 BC.
In the face of this panic, the Romans resolved to appoint a praetor maximus, or dictator, as the office came to be called, from whom there should be no right of appeal, for the duration of the emergency.
[ii] Alarmed by this development, the Sabines sent envoys to Rome to negotiate peace, while the Latins were not yet ready for war, and thus the dictator and magister equitum were able to lay down their office without taking the field.
Although the dictator commanded the entire army, his technical title was magister populi, or "master of the infantry", while the cavalry was entrusted to his lieutenant.
As with other magistrates, the lictors of the magister equitum were expected to remove the axes from their fasces when entering the Pomerium, an area of the city of Rome that was considered sacred.
[7] In the later republic, it was common for the magister equitum to be chosen from men of praetorian rank; that is, from those who had held the praetorship, but who had not yet been elected consul.
[1] Through the course of the fourth century BC, which saw the institution of the praetorship and the admission of the plebeians to the higher offices of state, the need to resort to extraordinary magistrates such as the dictator and magister equitum for military emergencies declined, and they were increasingly employed for ceremonial purposes.
The last dictator to take the field was Marcus Junius Pera in 216 BC, during the Second Punic War, with Tiberius Sempronius Gracchus as his master of the horse.
[15] Lepidus was the last magister equitum to hold military command, although he was neither the last nominated nor the last initiated; with the intention of beginning a new campaign in the autumn of 43, Caesar nominated his nephew, Gaius Octavius, magister equitum in advance, making the future emperor the last master of the horse to enter into office, although he never received his command.