The spread of Islam began in Sasanian Iran in the seventh century, and over several centuries extended eastwards from across the ranges of the Hindu Kush through to the Indian subcontinent. Most of the inhabitants of present-day Afghanistan, Pakistan, and areas of northern India were converted during this period to Sunni Islam, and indeed medieval Iran was also predominantly a Sunni territory, though the Shiʿa tradition took hold there in the long run, with important political and cultural conse…