From Islamic State: The Digital Caliphate, by Abdel Bari Atwan, pg 137:
Islamic State battalions and units are extremely adaptable to changing situations and new developments on the ground, and field commanders are given total autonomy in implementing the operations they are charged with. This flexibility, and confident delegation, makes IS’s war effort extremely effective and frees up the Military Council to concentrate on the overall strategy, rather than the detail of individual battles. Its opportunistic ‘liquid’ structure makes it difficult for global intelligence networks to get solid information and tangible details in order to counter and preempt Islamic State operations. Thus Islamic State always has the advantage of surprise and is able to seize opportunities as and when they arise. Rather than ‘fight to the death’, its brigades will slip away from a battle they are clearly not going to win, regrouping in a more advantageous location – a tactic successfully employed for many years by the al- Qa‘ida network. In January 2015, for example, with the US- led alliance bombarding Islamic State targets in Iraq, the Military Council decided to redeploy its efforts to Syria. Fighters inside Iraq were ordered to lie low (mostly in cities where it is harder for war planes to strike without significant ‘collateral damage’) while battalions and sleeper cells in Syria were reactivated. As a result, the group doubled the territory under its control in Syria between August 2014 and January 2015.33
Mark Carrigan 