It is also part of a longer trend predating the spread of Islam. Like other settled, agrarian societies in history, those in the Indian subcontinent have been attacked by nomadic tribes throughout its long history. In evaluating the impact of Islam on the subcontinent, one must note that the northwestern subcontinent was a frequent target of tribes raiding from Central Asia in the pre-Islamic era. In that sense, the Muslim intrusions and later Muslim invasions were not dissimilar to those of the earlier invasions during the 1st millennium.