Historians argue over the nature of the Roman army's collapse.
Some believe that the Roman empire's military adapted in response to changing conditions when the empire's leaders realized that conquest and expansion were no longer viable options due to limited economic and natural resources.
The Roman army initially focused on securing their borders and building forts and walls around cities, but this proved to be ineffective as invaders were able to fight their way into the cities.
Priority shifted to defense of frontier areas with Germanic troops from outside the empire.
The military then focused on the development of mobile units who fought enemies as they invaded the empire.
While some believed that a military composed of peasants and mercenaries was not as loyal as the traditional Roman army, others have contended that splitting the empire between Rome in the West and Constantinople in the East and adopting Christianity as the state religion profoundly affected all elements of social and military life.
Christianity questioned the divinity of the emperor which means he had less authority.
Constantinople had more resources and was easier to defend.
Some scholars point out that inflation and poorly equipped units stood little chance of stopping invaders so peasants were commanded by professional soldiers rather than military governors so that they could become more agile.
Thus, the Roman Empire became fragmented as its enemies became more powerful.
Lack of resources and inflation contributed to the eventual collapse of the Roman civilization and the development of smaller states.