As a matter of fact, the decline and collapse of the Ummayyad Caliphate was due to these reasons respectively:-
1- Economic hardships.
2- Delay of wages of military personnels
3- Expansion; to the limits where the Ummayyd couldn't keep their adversaries contained.
4- And most importantly, The Abbasid Revolt which promised a drastic reform, unity, and strength.
Here I must stop for a laugh at this ^
I understand that Persians in general can't take history as it is. Many are so desperate, endlessly try to manipulate history to feed their " Persian " Ego. The Abbasid Movement which was formed by multiethnic groups never distinguished between a white and a black person whatsoever.
I think you must have seen that " Persian " Army while you were asleep - because there were no such thing as Persia - back in the day when the Abbasid Movement was formed -
I'm also sure that you saw that " Persian " Commander of yours while you were asleep - because the Abbasid's Army encircle the Umayyad's from every angle -
Was it in the same manner the second Caliph did to your King with his crown? Being torn into pieces

The Buyid themselves were casted out in the middle of darkness by the Mongols!!!
It is interesting to see how some Iranians are willing to go to make them feel better.
Take history in the same way everybody takes it. History distortion courses through your blood.
@
al-Hasani @
Bubblegum Crisis @
Arabian Legend @
Talon