The Roman Empire began to slowly decline due to a series of plague. The population decline made it harder for Roman Emperors to control their armies and raise taxes.
Over expansion made it hard for the emperors to support the empire, creating instability and an uncontrollable empire. This put problems on other things such as the military control.
The numerous invasions had the Roman Empire crushed. Roman soldiers were loyal to their military leaders, not necessarily the emperor. The split of the empire into two parts weakened the empire as well.
Other reasons for the Fall of the Roman Empire: Christianity was less tolerant of other cultures and religions. The split of the empire into two parts weakened the empire. A failing economy High taxes Romans became lazy and comfortable