Economy
Constant invasions started a domino effect on the Roman Economy. The cost of defending the empire lead to overspending on the military, which lead to heavy taxation and unemployment among its people.
Barbarians
The Barbarians that were successful, which included Germanic tribes and the Huns, learned military tactics of the empire. They were treated cruelly by the Romans, and once the empire became unstable, they jumped at the chance to pursue their agendas.
Military
With the empire growing at a rapid pace, they needed a military that could keep up, so they recruited barbarians. The unfaithful barbarians weakened the force, along with civil unrest and the halt of technological advancement.
Inflation
With no regard to the empire’s failing economy, the leaders of Rome drained the treasury on personal luxuries. Along with the depleted treasury, Rome was no longer conquering, and the flow of gold and silver stopped. When the economic crisis hit, the government minted new coins, but they had little gold in them, which made them less valuable.
Christianity
The introduction of Christianity to the Romans shifted the glory and worship from the state and its emperors, to God. With the state already in political turmoil and confusion, the involvement of church leaders did not improve the situation.
Health
The impoverished of Rome lived in desolate conditions, dealing with war and revolts, plagues, famine and natural disasters. The wealthy led lavish lives, developing systems to supply the marble rooms in their houses with water. Unfortunately, this was done with lead pipes.
Civil War
After the division of the empire into East and West, there was constant civil war. When the two states weren’t murdering their own leaders, they were incessantly fighting over resources, territory, and military aid.
Population
In order to keep up with the empires expansion, slaves were imported for labor building roads, docks and aqueducts. Slaves made up 60% of the empire’s urban population and when the empire ceased to expand, they had nowhere to return.
Overexpansion
Rome had grown so large, it was unable to provide enough goods to everyone and during invasions, were unable to communicate quickly or effectively enough. It was decided by Diocletian that the empire was too large for one ruler, so he split it into East and West, keeping East and giving the West to Maximian.
Corruption
Rome never established a system for electing its leaders, and with the profits it was receiving from war, everyone wanted a shot at the throne. This led to auctioning off positions to the highest bidder, illegal behavior among governors and generals, and those in power using the war riches to live in the lap of luxury.