First a "one stop" explanation from the BBC 'Islamic State group: The full story' and then a Salon article, which includes a Q&A with Will McCants; with a sub-title:
Simple-minded analysis of religion, history and our challenge in the Middle East leads to the wrong policy choices
The BBC:http://www.bbc.co.uk/news/world-middle-east-35695648

Salon:http://www.salon.com/2016/03/19/the_islamic_state_would_not_be_with_us_today_if_th e_united_states
_hadnt_invaded_iraq_islam_religion_and_the_intervi ew_donald_trump_and
_ted_cruz_must_read_now/

Thread created for maximum visibility; there is a main thread already on watching ISIS.