What was the main reason why the han dynasty and the western roman empire collapsed?

The main reason why the Han dynasty and the Western Roman Empire collapsed was immigration. The Hun dynasty suffered emigration of nomadic people to northern China and the Western Roman Empire was invaded by  Germanic invaders (Vandals, Ostrogoths, and Visigoths).

All three fell because of internal conflicts and external invaders.

1. it was caused by feudalism

2. charlemagne used christianity as a major cornerstone for his rule. cloisters were built all over the empire to act as centers for the (re)population of the vast european wilderness.

3. the europeans feared the next raid every time chetak

i'm not so sure about #6. but i hope i you with the first 3.

a) "how old man winter was driven back " and "ormuzd and arimanes"

hope it