The Roman army evolved over time from using the phalanx formation to developing new battle techniques using three battle lines. They eventually standardized their army into legions, each led by a legatus for around 3 years. The Roman military hierarchy had senior officers like the dux and legatus legionis, mid-level officers such as the primus pilus and centurious, and lowest officers like the legionarii and tirons. Modern militaries continue to be influenced by the Roman system, using a ranked structure and separating forces into army and navy branches.