* Citizen militias: The Roman army was primarily composed of citizen soldiers who served for a fixed term of one to two years. These soldiers were organized into legions, which were large infantry formations of around 5,000 men. The legions were commanded by elected magistrates called consuls, who were assisted by a staff of military tribunes.
* Professionalization of the army: As Rome expanded its territory and became a more powerful empire, the need for a more professional army became apparent. In the 2nd century BC, the Roman army began to recruit more and more soldiers from the lower classes, who were willing to serve for longer periods of time in exchange for pay.
* Marian Reforms: In 107 BC, the Roman general Gaius Marius introduced a number of reforms that transformed the Roman army into a more professional and effective fighting force. Marius standardized the equipment of the soldiers, improved their training, and increased the size of the legions to 6,000 men. He also created a system of permanent military camps, which allowed the soldiers to train year-round and to be deployed more quickly in times of war.
Late Republic (200–31 BC)
* Rise of mercenary armies: During the late Republic, the Roman army became increasingly reliant on mercenary troops, who were often more skilled and experienced than the citizen soldiers. This was due in part to the growing urbanization of Rome, which made it difficult to recruit enough citizens for the army.
* Political interference: The Roman army also became increasingly politicized during the late Republic. Generals often used their armies to support their political ambitions, and civil wars became more frequent. This led to a decline in the discipline and morale of the army.
Early Empire (31 BC–284 AD)
* Augustan Reforms: After the establishment of the Roman Empire, the emperor Augustus introduced a number of reforms that further professionalized the Roman army. He created a standing army of around 150,000 men, which was divided into legions, auxiliary units, and cavalry units. The legions were commanded by experienced generals who were appointed by the emperor.
* Pax Romana: The Pax Romana, or Roman Peace, was a period of relative peace and stability that lasted for over two centuries. During this time, the Roman army was used primarily to defend the borders of the empire and to maintain internal order.
* Crisis of the Third Century: The Crisis of the Third Century was a period of political instability and civil war that lasted for nearly fifty years. During this time, the Roman army was weakened by a series of defeats, and the empire was threatened by invasions from Germanic tribes.
Late Empire (284–476 AD)
* Barbarian invasions: The late Roman Empire was increasingly threatened by invasions from Germanic tribes. In 376 AD, the Visigoths defeated a Roman army at the Battle of Adrianople and sacked Rome in 410 AD. The Roman army was unable to stop these invasions, and the western Roman Empire collapsed in 476 AD.
* Recruitment of barbarians: In order to bolster their armies, the Romans began to recruit more and more barbarians from the tribes that were threatening the empire. These barbarians often served as auxiliaries, or support troops, but they also eventually rose to command positions in the Roman army.
* Foederati: The Romans also made alliances with barbarian tribes, who were given land in exchange for military service. These tribes were known as foederati, or allies, and they played an increasingly important role in the defense of the empire.