Germanic Barbarian Invasionhttp://www.angelfire.com/darkside/sjhscult/notes/unit1/fall_rome.htm
For centuries, Germanic tribees had been a constant threat to the Empire. SInce Trajan expanded the Roman Empire to its greatest extent, the military and border control to protect the whole border. Doing so, the protection grew thin. By this time in 476 the Germanic Barbarians had destroyed the western part of the Empire,
Economic DeclineWhat are the four causes of the downfall of the Guptas?The income of the Gupta Government declined after later emperors made large scale land grants. This made it difficult for the rulers to pay back the owed expences. Also, the import of gold and silver stopped, which lead to weak foreign trade.
Han Empire Defeats Xiongnu Statehttp://asianhistory.about.com/od/ancientchina/f/Why-Did-Han-China-Collapse.htm
From 133BC to 89CE, the Chinese and Xiongnu fought over land in the western regions of China, in a war known as the Sino- Xiongnu Wars. This area was critical because the silk road ran through it and carried the goods to Han cities. In 89CE, the Hans defeated the Xiongnu state, but it came at a very high cost. This helped dramatically tare down the Han government.
Diocletian Becomes RulerSource: Notes
In 284 CE, Diocletian became emperor of Rome. He tried many political reforms that did not work. Some reforms that he tried were setting price limits. If a person went beyond their limits, they were put to death. Another reform was forcing workers to stay with their jobs until death.
Hepthalites Invade Guptan Empirehttp://en.wikipedia.org/wiki/Gupta_Empire#Decline_of_the_empire
In the 480's, the Hepthalites broke through Guptan defenses in the northwestern part of the empire. After that, most of the northwest was overrun by the Huna by 500.