Buddy Muhammad Bin Qasim invaded under Ummayad Caliphate, the invasion was cuz of strategic and trade reasons, plus the local population had turned against Sindhi ruler Raja Dahir, the notorious king who married his own sisters, they found it a brilliant opportunity, Ummayad empire was huge on one end it had sindh and multan, on the other it had Lisbon in Portugal, as far as the plunder is concerned this has always happened throughout human history, in europe, in africa or in asia, the victorious always did plunder the wealth of defeated army, Nadir Shah u r referring to also defeated and killed Ottoman Caliphate's Grand Vazir Osman Topal Pasha and then plundered Dehli, however the invasions under Rashidun Caliphate were different, that is the first 4 caliphs and companions of Prophet Muhammad (S.A.W), when Sassanids were defeated and Pakistan's baluchistan was captured, even the people of Sassanid empire and Byzantine empire rejoiced when under Caliph Umar (R) their lands were liberated, cuz their reign wasnt guided by lust for materialism rather spiritualism, after those 4 caliphs (around 30 years after death of Prophet (S.A.W) arab nationalism and materialistic kingdoms (just like any other kingdom) like ummayads started