Al Qaeda was originally envisioned as a kind of Sunni foreign legion, which would defend Muslim lands from Western occupation….
Bin Laden had asked Zarqawi [founder of ISIS] to merge his forces with Al Qaeda, in 2000, but Zarqawi had a different goal in mind. He hoped to provoke an Islamic civil war, and, for his purposes, there was no better venue than the fractured state of Iraq, which sits astride the Sunni-Shiite fault line….Violent attacks would create a network of “regions of savagery,” which would multiply as the forces of the state wither away, and cause people to submit to the will of the invading Islamist force….[A] broad civil war within Islam would lead to a fundamentalist Sunni caliphate.
In other words, Al Qaeda is focussed on expelling the West from the Middle East but ISIS is focussed on creating a Sunni Islamic superstate. Hence, Al Qaeda attacks the West but ISIS attacks Shiites. This leads to different “realist” policy prescriptions – self-interest implies attacking Al Qaeda but not necessarily ISIS. It leads to the same neocon policy prescriptions – there will be a humanitarian crisis from civil war and democracy (to the extent Iraq is a democracy) is threatened. Hence, send in the troops say Kristol and Kagan, just as they did before Gulf War II.