How did mercenaries weaken the roman military

During the decline, the Roman army had been severely weakened to the point where invaders that could have previously been defeated by the Empire were eventually able to destroy it. Ultimately, the Roman Empire collapsed because of "the general failure of its armies to perform the tasks that were required of them." One reason for the breakdown of the army was that the Roman legions would fight in civil wars over their choices for Emperor because they were unhappy with the weaker Emperors that were frequent in the later Roman Empire. "These struggles served as an irresistible invitation to German and other enemies to break into the distracted provinces." Although the civil wars served as a distraction to the Roman army, the wars also had a detrimental effect on the army because they damaged its manpower, both in quality and numbers.

The Roman army during this time was divided into the high quality soldiers, called the field force, and the lower quality soldiers, referred to as the frontier force, who did not fight as much as the field force. The frontier force was mainly deployed to local garrisons that were not as volatile as the places where the field forces fought. Because of the civil wars between the legions and the battles against outside invaders and enemies of the Western Roman Empire, the field force was considerably weakened and greatly reduced in number. The field force had to take second-rate soldiers from the frontier force, thus lowering the quality of the army. The waning quality of the Roman army was not its only concern; there were also not a sufficient number of new soldiers being drafted.  all the info is in there thank litpick.com i didnt write this