How to define the exact reason:
When Commodus became emperor in ad 180, the age of the good emperors came to an end, and soon the Roman Empire experienced far worse leadership.
Between ad 193 and 235 a series of rulers known as the Severan dynasty ruled Rome, but for much of that time civil war continued in many areas.
Septimius tried to keep soldiers loyal by raising their annual pay and by relaxing military discipline. He permitted legionaries who were on active duty to marry, farm their own land, and live in cities rather than in camp. He trusted the army so much that he gave soldiers numerous administrative tasks such as tax collection, which lessened military readiness...
Military Anarchy...
The Severan Age was a time of turmoil, but Rome remained a large empire with an impressive system of law, food production, commerce, and frontier defense. Its fatal weakness lay in its lack of a constitution. After Septimius Severus, all power derived from the army, which claimed to represent the Roman people. Earlier civil wars had shown that legions would support their own commanders in the hope of rewards. For 50 years generals caused incredible destruction in their quest for power, but their efforts were largely in vain. Between 235 and 284, the troops acclaimed about 20 “emperors” and another 30 “pretenders,” although the two groups only differed in that the emperors briefly managed to control the city of Rome. Only one of these emperors died of natural causes, so the imperial throne was a dangerous prize...
Diocletian...
The military anarchy of past regimes had caused economic collapse as rival emperors produced worthless coinage to pay their troops. Diocletian instituted broad economic reforms in an attempt to restore value to the currency and to control runaway inflation. He also established a new system of taxation to finance the imperial budget. Since inflation threatened people on fixed salaries, including most members of the army and the bureaucracy, Diocletian issued a decree that attempted to set maximum prices across the empire for everything from onions to haircuts to Chinese silk. It became known as his famous Edict on Prices...
Diocletian was not successful in all his individual economic policies, but through years of unremitting effort he restored the economic health of the empire that had suffered from half a century of reckless expenditure...
Constantine the Great...
Authoritarian rule permeated every aspect of Roman life as the government bound farmers to their land and craftspeople to their trade. The government required the sons of bakers or shipbuilders to follow their fathers’ careers. The emperors even established a secret police, and the old unregulated economic system yielded to a planned economy. The emperors often appealed to the public good when they suppressed individual rights, requisitioned goods, or increased taxes. In the words of one writer of the period, the empire became a prison.
Fall of the Western Empire...
Several factors explain why the Roman state collapsed in the west and survived in Constantinople for another 1,000 years...
The demands of the military and the growing bureaucracy forced the government to seek more income.
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