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In page Islamic terrorism:

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Some Muslim scholars assert that extremism within Islam goes back to the Kharijites who existed in the 7th century. From their essentially political position, they developed extreme doctrines that set them apart from both mainstream Sunni and Shi'a Muslims. The Kharijites were particularly noted for adopting a radical approach to takfir, whereby they declared that other Muslims were unbelievers and therefore worthy of death.[1][2]