Besides the dark side of the incident of Karbala, we see that Muslim rule extended to Spain & Sindh, during Umayyad period. What were the factors that resulted in such victories?
Well even the Qarbala incidence was foretold to the Prophet (Sallalahu Alaihi Wassalam), and like there's a shaer "Islam zinda hota hay her qarbala kay baad".
Its my personal opinion that since the prophets family members (Sallalahu Alaihi Wassalam) didnt get involved in the politics after Hazrat Ali (RA) they rendered a bigger service to Islam by imparting their knowledge and bringing non muslims to Islam.
Anyhow interesting topic, what do you thinik were the reasons for that?
In my opinion its better administration that resulted big victories during that period. Although Ummayad rule ended 750 AD (started in 661), but the Umayyad dynasity in Spain continued till 12th Century.
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Shrewd people like Hajjaj Bin Yousuf also played important rule in extending boundaries