The following article is a long, detailed discussion of ISIS’ ideology, how it influences their policies, and what the implications are for the West’s attempts to deal with the threat they pose.
“The Islamic State, also known as the Islamic State of Iraq and al-Sham (ISIS), follows a distinctive variety of Islam whose beliefs about the path to the Day of Judgment matter to its strategy, and can help the West know its enemy and predict its behavior.”
There are a number of things to consider when reading this article. First, how accurate is the description of the groups ideology. Although the article does give a good account of how ISIS explains its ideological position, there have been critiques of the way the article deals with Islamic thought (see: http://thinkprogress.org/world/2015/02/18/3624121/atlantic-gets-dangerously-wrong-isis-islam/).
Second, are ISIS’ goals derived from their ideology, or does ideology just provide legitimacy to mundane political goals after the fact.
Third, are there ideological schisms within ISIS and if so, what could they mean for ISIS’ policies? Ideological movements (communists in the USSR and Khomeini’s Islamist followers in Iran) looked monolithic at first but it did not take long before deep divisions became apparent. We actually know very little about what is happening inside of ISIS right now. It is possible that ideological divisions may lead to moderation, with soft-liners diluting the ideological zeal of the movement. But it is also possible that divisions will lead to more extreme behavior as competing factions try to out-bid each other for ideological legitimacy.
Fourth, there is a tendency to see ideologies as static, when in fact they may evolve over time. It may evolve because of internal divisions and debates. Or, it may evolve as the organizational needs of the movement change. Right now, ISIS is in its formative stages. The extremism and violence provides the group with an identity, improves solidarity and intimidates internal dissent.However, if ISIS survives over the long term, it will likely begin to function more and more like a regular state, with regular institutions etc… In the Iranian case, the day-to-day demands of running a state and keeping it safe had an important moderating effect on the ideology of the leadership. This may, or may not happen with ISIS.