There has been a lot of myth about Mohammed bin Qassim in Pakistan history books. There is no denying his military achievements, but we have to understand that the real personality behind the conquest of Sind was Hajjaj Bin Yusuf Al Saqafi.
Mohammed bin Qassim was appointed primarily because he was related to Hajjaj (Nephew /son in law?) and therefore could be trusted implicitly. Fortunes of Mohammed bin Qassim were closely linked to the rise of Hajjaj bin Yusuf.
Hajjaj was a school teacher from Taif who became an ardent supporter of the Umayyad. He proved himself to be a ruthless and ferocious army commander. Many people are not aware and Muslim historians deliberately ignore that fact that for 9 years there were 2 Caliphs in the early Islamic State.
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“Ibne Zubair was Khalifah from the year 63 AH to 73 AH and that Addul Mailk was also Khalifa in Syria.”
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http://www.salafipublications.com/sps/downloads/pdf/SRH020001.pdf
After the transfer of power from Abu Sufian family to the Marwan branch of the Umayyad, Abdulla ibne Zubair (RA) held the title and power of Kahlifah over the Hijaz and much of the area in the south. Hajjaj crushed the rebellion in 73 AD, Kaaba itself, was attacked and destroyed in the process and head of Ibne Zubair (RA) hung from the gate of Kaaba until it rotted.
Hajjaj was later appointed Governor of Iraq and all lands to the East. It said that Hajjaj completely subjugated the Arabian peninsula, Iraq and Khurasan to the Umayyad rule but also killed more than 100,000 mostly Arab Muslims in the process. Apparently even Anas ibne Malik (RA) , last of the known living Sahabi (died 103 AH) because of his suspected support of the rebels, was forced to wear a collar round his neck.
Power of Hajjaj grew to such an extent that he even issued coins in his name. This however resulted in jealousy among the Umayyad royals. Hajjaj died in 714 AD. As soon as Suleiman bin Abdul Malik became Caliph in 715 AD, he recalled the two generals Mohammed Bin Qassim & Quitaba bin Muslim ; considered favourites of Hajjaj and had them killed.