During those times, some Muslim traders living in Ceylon died and the ruler of Ceylon sent their widows and orphans back to Baghdad. They made their journey by sea. The King of Ceylon also sent many valuable presents for Walid and Hajjaj. As the eight-ship caravan passed by the seaport of Daibul, Sindhi pirates looted it and took the women and children prisoner. When news of this attack reached Hajjaj, he demanded that Dahir return the Muslim captives and the looted items. He also demanded that the culprits be punished. Dahir replied that he had no control over the pirates and was, therefore, powerless to rebuke them. On this Hajjaj decided to invade Sindh. Two small expeditions sent by him failed to accomplish their goal. Thus, in order to free the prisoners and to punish the guilty party, Hajjaj decided to undertake a huge offensive against Dahir, who was patronizing the pirates.

In 712, Hajjaj sent 6,000 select Syrian and Iraqi soldiers, a camel corps of equal strength and a baggage train of 3,000 camels to Sindh under the command of his nephew and son in-law, Imad-ud-din Muhammad bin Qasim, a young boy of just seventeen years. He also had a 'manjaniq', or catapult, which was operated by 500 men and could throw large stones a great distance. On his way the governor of Makran, who provided him with additional forces, joined him. Also, a good number of Jats and Meds, who had suffered at the hands of native rulers, joined the Arab forces.