Causes of the Arab Invasion:
The first and foremost reason of the invasion of Sindh was to plunder the wealth of India. The Arabs had commercial relations with India since long. Indian rulers had not only welcomed them but also provided several facilities to them, but after their conversion to Islam, a change appeared in their attitude and instead of increasing their trade relations, they began to think of plundering the rich coasts of India.
The Arabs also wanted to expand Islam. By now they had extended the Muslim empire up to Kabul in the east and they intended to attain victory in Sindh in order to preach the religion of the prophet in India. India was a country where mostly Hindus lived; hence the aim of the followers of Islam was to spread Islam in India.
Immediate Cause of Invasion:
A party of Arabs was returning from the Malabar Coast with gifts for Caliph Walid I at Damascus and his co-viceroy Hajjaj at Basrah. It was plundered by pirates near the mouth of the Indus and the Arabs were detained at the port of Debal (Karachi). A demand for the restitution was made to the ruler of Sindh, but it was met with evasion.
Hence a punitive expedition was at once dispatched by Hajjaj, but it could not succeed. Another was sent in its wake with no better results. Finally, in the autumn of 711 A.D. the enraged governor of Iraq dispatched his own nephew and son-in-law, Imaduddin Muhammad-bin-Kasim, with a powerful army consisting of 6000 Syrian horses, an equal number of Iraqi camels and 3000 Bactrian baggage animals. Muhammad-bin-Qasim was a youth of seventeen but his success showed that he had an old head over young shoulders.