The conquest of Sindh, located in today’s Pakistan, happened in stages. During the Caliphate of Omar ibn al Khattab (r), Muslim armies approached the coast of Makran, but Omar (r) withdrew the troops in response to reports of a harsh and inhospitable terrain. Emir Muawiya subdued eastern Afghanistan and the Northwest Frontier areas. However, it was not until the reign of Walid I (705-713) that much of what is today Pakistan was brought under Muslim rule.

From pre-Islamic times, there was a brisk trade between the eastern coast of the Arabian Peninsula and the western coast of India and Sri Lanka. Ships rode the eastern monsoons to the coast of Malabar and Sri Lanka to pick up spices and returned home riding on the western monsoons. Spices were in great demand throughout West Asia, North Africa and southern Europe and transactions were extremely profitable. This trade continued to thrive and expand with the advent of Muslim rule in West Asia and North Africa. It was through these merchants that Islam was first introduced into Kerala in southwestern India and Sri Lanka, located near the tip of India.

Sindh was notorious for its pirates in those times. These pirates would wait in ambush for merchant ships on the coast of Sindh and would raid them for booty. In the fateful year 707, these pirates attacked one of the Muslim merchant ships sailing back from Sri Lanka to the Persian Gulf. The men, women and children on board the ship were captured and taken inland to Sindh, where the Raja imprisoned them.

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source:

http://historyofislam.com/contents/the-age-of-faith/the-conquest-of-sindh/

http://300035.dotdothktrade.tech/en/artical/21275/conquest-of-sindh