How big was the Roman army at its peak? How did the size of the army change throughout history?
The Roman army was one of the largest and most formidable armies in human history due to its organization, discipline, and efficiency. At its peak in the 3rd century AD, the Roman army consisted of around 30 legions and a number of auxiliary regiments, which was about 450,000 troops in total. Before this period, the Roman army under Augustus had only around 25 legions (125,000 men), but as the empire grew in size and its borders became more threatened soldiers were hired from every corner of the empire in order to maintain peace and order.
Under the emperor Tiberius in the 1st century AD, the army in its entirety consisted of around 250,000 troops, which then grew and reached its peak under Septimius Severus in the 3rd century.
After the Severan reign, however, the many crises and civil wars of the Empire led to a sharp decline in army sizes, with the average number of troops being around 250-300,000. These numbers increased towards the end of the empire, however, particularly under Diocletian when forces rose to around 400,000 once more.
It is important to remember that these troops were never all in the same place at one time but instead were spread throughout the empire in order to maintain borders and public order.