Why did Muhammad bin Qasim attack on Sindh?
There are various reasons cited by Arab Historisns in Chachnama.
- Jat pirates plundered an Arab ship plying from Sri Lanka to Yemen. It carried the gifts from the Raja of Srilanka for caliph. This happened near the port of Debal.
- Pirates not only looted the money but they took away some Arab beauties travelling in the ship.
- Ummayads were expansionists ,before that three major attempts and 14 minor raids were carried out by Arabs to capture Sindh. This third reason is not given in Chachnamah.
- Raja Dahir gave shelter to Alafi Arabs who were hounded out by Ummayads.
- Raja also gave shelter to fugitive Iranian nobles from Persia.
It may be true that a ship might have been plundered by Jats of Sindh. But it is unlikely that they seized girls and violated them. Because first of all this is not the nature of Jats even if they were plunderers. Secondly, Arab traders never took their women to far away land in Ship. Rather one of the item of the trade used to be foreign girls who later would be sold to the markets of Basra, Damescus , Madina and Mecca. At the most Hindus would have rescued some *** girls boarded on the Arab ship and that would have irritated Caliph.
Before Quasim ,other Arab raids to Sindh failed. It s recorded in Chachnamah that “ if you attack Sindh and Makran with small army, it will be cut in to pieces by dacoits of Makran( Jats and Meds). If you send a huge army it would be perished with thirst”.
As per Farishta, the Muslim historian of 12 th AD , At the time of Quasim , Kandahar ,Ghajni and Kabul were ruled by Kushan Shahi Hindus. Professor Ahmed Hasan Dani has done extensive research in the area. He had discovered the name of the king of Kandhar during Quasims attack on Sindh as Vasudev.
Muslims tried to attack Sindh from Kandhar side but perished due to bravery of local king. Then Arabs sneaked between Ghajni and Kandahar and avoiding the direct confrontation with any royal army ,They endeavored to enter thru the Kikkan pass, but they were cut to pieces by Jats guarding the area.
Coming back to 711 AD. Caliph asked Raja Dahir to punish Sindhi pirates and compensate for the loss. Raja responded by stating that he had no control on the pirates so he cannot punish them. This infuriated the Caliph and he ordered Hajaj bin Yusuf the Governor of Iraq to punish the Rajah. Hajaj bin Yusuf sent strong Army of 50,000 under the command of 17 year old Muhammad bin Quasim, who was nephew and son in law of Hajaj bin Yusuf.
By 711, Arabs have captured Zarang province of Iran and small portion of western Makran. This facilitated the supply lines to the Arab Army. Smarting over the previous Arab failures of the area, Arabs befriended Meds and Jats of area thru money and promised them further share in the *** if they come along. Unfortunately Raja Dahir had humiliated Jats by depriving them of wearing traditional turban and using horses. He declared Jats as Shudras. Thus Jats unwillingly supported Quasim in his attack on Sindh. Jats later realized their Mistake when they saw the actions of Arabs to the captives of war ,specially women but that was too late.
I am not going in to details of war, as this is not asked in the question but succinct to say that split among the three martial tribes paved the way for easy capitulation of Sindh. Jats and Meds deserted Dahir, some Rajputs like Bhati, Lohani and Sumra also deserted. Raja Dahir has to fight with solely Brahmans and few loyal Rajputs.