It is true that Sindh was actually conquered by the Arabs in 712 A.D., but it does not mean that no efforts had been made earlier in that direction. We are told that in 636-637 A.D. during the Khalifat of Omar a naval expedition was sent for the conquest of Thana near Bombay, but it failed.
Another attempt was made in 644 A.D. by land through the Makran Coast into Western Sindh. The expedition was sent by Khalifa Osman under the leadership of Abdullah-bin-Amar. The latter conquered Sistan and then advanced towards Makran. He defeated the ruler of Makran and Sindh.
In spite of these victories, it was not considered worthwhile to annex Sindh. It was reported to the Khalifa that in Sindh “water is scarce, the fruits are poor; and the robbers are bold: if a few troops are sent, they will be slain, if many, they will starve”. In spite of difficulties, the Arabs continued to attack the frontiers of Sindh both by land and sea.
They concentrated their attacks on Kikan which was a hilly region round the Bolan Pass. Al-Haris won some success in 659 A.D. but was ultimately defeated and killed in 662 A.D. Another successful attempt was made by Al-Muhallab in 664 A.D. Abdullah made an unsuccessful attempt and he was killed. Ultimately, the Arabs were able to capture Makran or Baluchistan in the first decade of the 8th century A.D. and this prepared the way for the conquest of Sindh.