Many emperors after Marcus Aurelius were challenged by the military. This was because the military had become increasingly powerful and influential in the Roman Empire, and they had the ability to appoint and depose emperors. This was especially true during the Crisis of the Third Century, when the military was able to appoint and depose emperors at will. This led to a period of instability and civil war, as different factions of the military fought for control of the empire.
