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 sub-continent, one must note that the northwestern sub-continent was a frequent target of tribes raiding from Central Asia. In that sense, the Muslim intrusions and later Muslim invasions were not dissimilar to those of the earlier invasions during the 1st millennium. What does however, make the Muslim intrusions and later Muslim invasions different is that unlike the preceding invaders who assimilated into the prevalent social system, the successful Muslim conquerors retained their Islamic identity and created new legal and administrative systems that challenged and usually in many cases superseded the existing systems of social conduct and ethics, even influencing the non-Muslim rivals and common masses to a large extent, though non-Muslim population was left to their own laws and customs. They also introduced new cultural codes that in some ways were very different from the existing cultural codes. This led to the rise of a new Indian culture which was mixed in nature, though different from both the ancient Indian culture and later westernized modern Indian culture. At the same time it must be noted that overwhelming majority of Muslims in India are Indian natives converted to Islam. This factor also played an important role in the synthesis of cultures.