(8:15) Believers, whenever you encounter a hostile force of unbelievers, do not turn your backs to them in flight.
(8:16) For he who turns his back on them on such an occasion - except that it be for tactical reasons, or turning to join another company - he shall incur the wrath of Allah and Hell shall be his abode. It is an evil destination
comment: The Qur’an does not forbid orderly retreat under strong pressure from the enemy provided it is resorted to as a stratagem of war, for example seeking reinforcements or joining another party in the rear. What the Qur’an does forbid is disorderly flight produced by sheer cowardice and defeatism. Such a retreat takes place because the deserter holds his life dearer than his cause. Such cowardice has been characterized as one of those three major sins which, if committed, can be atoned for by no other good deed whatsoever. These three sins are: ascription of divinity to anyone or anything other than God, violation of the rights of parents, and flight from the battlefield during fighting in the way of God. (See al-Mundhiri, ‘Kitab al-Jihad’, ‘Bab al-Tarhib min al-Firar min al-Zahf’- Ed.) In another tradition the Prophet (peace be on him) has mentioned seven deadly sins which totally ruin a man's Next Life. One of these is flight from the battlefield in an encounter between Islam and Unbelief. (Muslim, ‘K. al-lman’, ‘Bab al-Kabi’ir wa Akbaruha’; Bukhari, ‘K. al-Wasaya’, ‘bab - fi Qawl Allah - Tala: inna al-ladhina Ya’kuluna Amwal al-Yatami Zulman’ - Ed.) This has been declared a deadly sin because in addition to being an act of sheer cowardice, it demoralizes others and can generate demoralization which can have disastrous consequences. An individual soldier's desertion might cause a whole platoon, or even a whole regiment, and ultimately the whole army, to take flight. For once a soldier flees in panic, it is hard to control the others.
http://www.islamicstudies.info/tafheem.php?sura=8&verse=11
This means that all returnees from Syria etc. even if they want to depart from IS/Daesh will face a conflict between their faith and the fact that they have departed from the battlefield. If they meet a charismatic recruiter they will find it very difficult to say no to him. Many are driven by a belief that they have been a sinner earlier and the only way to go to Paradise is to be martyred (= killed for Allah) We have to prevent them from doing that here, and this will not be easy.