By the end of the third century A.D., the empire was getting plagued by civil war and barbarian invasions. Eventually the old legions became disorganized and spread out. Cohorts were disbanded and others were sent to various regions to fill gaps in the defense. Legions were spread out across the empire's now huge territory. The empire had now expanded and subjugated many territories and people, and with that came the recruitment of non-Romans into the army. It helped appease and control the newly won areas by offering citizenship at the end of a military career.

The practice of allowing non-citizens to join the army began after the Marius Reforms. “The link between citizenship and military service” was now severed. Especially during the civil war period when many legions were raised, men were recruited to the army even though they lacked the essential requirement: citizenship. Especially when the Empire became so large that it was divided into the East and West Empires, “the imperial authorities found it difficult to recruit citizens over large areas of the empire”. This meant that when a barbarian or other enemy threat arose in different areas of the east and west empire, it was easier to raise barbarian armies and use ally troops as a large chunk of the army, instead of having a “one size fits all” professional army across the whole empire. It was not that the Roman Empire's citizen population was declining during this period. On the contrary, citizenship had never been higher, and there could have been a steady flow of Roman citizens into the army if the imperial authorities did not rely so much on mercenaries and barbarians. This in the end, as we will see, directly contributed to the fall of the Roman Empire.

From the start of the empire to the end, the army steadily became more culturally diversified. By the third and fourth century, the army increasingly relied on employed barbarians and allies to do the fighting, instead of training the vast population of Roman citizens. The army became more of a “mercenary” body, as the number of allied troops outnumbered the Roman ones. This diluted the once proud and powerful army into a mixture of unreliable troops.