Answer:
Read below
Explanation:
1: Barbarians kept on attacking
2: Internal stability like the killing of Julis Ceaser
3: Ethnic groups inside the Roman Empire that wanted to be free
4: People were hungry for power
5: Failing economic system
6: The Roman Empire being split in two
7: Some people didn't want to change there religion
8: Rise Of Islam ( later years )
9: Different cultures
The answer to your question is "unfair taxes"
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There have been many different forces that have linked Arab nations, but most of these links have come from military dominance in one way or another.