Beginning in 2004, the precursor to the Islamic State (IS) – Al-Qaeda in Iraq – antagonized allies and foes by its brutal violence. Criticized by Al-Qaeda, fought by local Sunni tribes, and hunted by US special forces, the IS reached its nadir in 2009. The excessive brutality did not hinder the comeback of the organization, nor did it leave the IS without supporters in the protracted territorial war from 2014 to 2017. To understand IS and what allows it to remain a part of Iraq, this chapter seeks to answer the following questions: How could the IS successfully occupy large swaths of Iraq less than four years after its nadir in 2009? And what caused the territorial war against the IS to drag out for more than three years? The anti-IS coalition was numerically superior and benefited from an air campaign that wreaked havoc on IS units. Finally, the phoenix character of the IS forces us to ask the uncomfortable question: Could the IS once again change its tactics and conquer territory?