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The legion — legio, from the verb legere, to raise (a troop) — was the basic unit of the Roman army from the time of ancient Rome until the end of the Roman Empire. During the twelve centuries (from 753 BC to 476), the numbers and composition of the Roman legions varied greatly. These changes did not only concern the legions composed of legionaries but also the auxiliaries, the praetorian cohorts, the urban cohorts and the Roman navy. Thanks to its many military successes both in the Republican era and later in the Imperial era, the legion was long considered a model to follow in terms of efficiency and potential in terms of military tactics.