First Muslim conquest of the Indian subcontinent and Md. bn Quasim

Bin Quasim leading hos troop.en.wikipedia.ord.

Muhammad bn Qasim (695 –715 CE)hi.wikipedia.org

Muslim conquest of the Indian subcontinent began during the early 8th century. In those days sea pirates were a menace to several  countries and the sea trading was very much restricted. Sea pirates, unable to get a bounty, would sometimes make raids into coastal areas and indulge in rampage, looting, kidnapping, etc causing destruction. This kind of problem was quite frequent in west Asia and also along the Baluchi, Kutchi coasts.

Muhammad bin Qasim, (December 695 –18 July 715) a member of the Thaqeef tribe of the Taif region, was  an Arab general for the Umayyad Caliphate and was well trained in warfare and governance by his  paternal uncle.Under Hajjaj's patronage, Muhammad bin Qasim was made governor of Persia.
Umayyad interest in the region occurred because of frequent attacks on Arab ships from pirates of Sindh province ruled by Raja Dahar.

During Hajjaj's governorship, the sea pirates mostly the Mids (are Muslims and like many other Makran communities, they are also divided along sectarian lines) of Debal in one of their raids had kidnapped Arab Muslim widows and children returning from Sri Lanka to Arabia. The Hindu king of the Sind province not only protected the pirates but also refused to release the hostages. It angered Hajjaj and he dispatched a powerful well trained army of 6,000 cavalry under the command of Muhammad bin-Qasim in 712 CE.

When Muhammad Bin Kasim invaded Sind in 711 AD, Buddhism was the main religion and the rulers had  no resistance whatsoever against the fire and steel power. Unstoppable Muhammad triumphantly marched into the country, capturing  Sehwan, Brahmanadabad, Alor and Multan,etc one after the other in quick succession. In less than a year and a half, the far-flung Hindu kingdom was crushed, the great civilization was almost lost. Under the misrule of raiders from NW, Sind entered the worst period of its history marked by tyranny and suppression of other religious faiths. Mosques sprang up in the place of temples at locations like  Debal, the Nairun and Aror.

In those days in many NW countries  and also in other countries as well, expansion of kingdom was an important part of the rulers. The ruler would raid the rich countries near by and plunder and loot as much as they could to improve the wealth of their countries. A big portion of the treasures was used for army mobilization,etc.

In the west Asian countries, through travelers, the rulers came to know of the wealth of India in those days and the valuable treasures stored in the Hindu temples, especially in north and western parts of India. Somnath temple complex in Gujarat was one of a few richest places of worship in the world. Junayad, the Arab Governor of Sind set his eye on the rich temple and in 725 CE  sent his  armies to destroy and plunder the temple of all available treasures at any cost. The massive raid was a success and the people and ruler at that time were caught unawares by the massive raid by the dangerous, unscrupulous raiders. The outcome of the raid was unfortunate. What was once a beautiful place of worship, a repository of vast treasures, over a short period became a mound of rubbles and rubbish with almost all treasures gone for good. Undeterred and unmoved, the Gurjara Pratihara king Nagabhata II rebuilt the temple in 815 - a large  grand structure of red sandstone. 

Quasim's  successful raid in the subcontinent encouraged other Warriors of Afghan and Turk origin to follow suite.
Back in Persia Qasim was killed because of a family feud with the new governor.

Ref:
http://en.wikipedia.org/wiki/Muhammad_bin_Qasim