(And whoever turns his back to them on such a day -- unless it be a stratagem of war...)
The Ayah says, whoever flees from the enemy by way of planning to pretend that he is afraid of the enemy, so that they follow him and he takes the chance and returns to kill the enemy, then there is no sin on him. This is the explanation of Sa`id bin Jubayr and As-Suddi. Ad-Dahhak also commented, "Whoever went ahead of his fellow Muslims to investigate the strength of the enemy and make use of it,
أَوْ مُتَحَيِّزاً إِلَى فِئَةٍ
(or to retreat to a troop (of his own)), meaning he leaves from here to another troop of Muslims to assist them or be assisted by them. So that is allowed for him, or even during the battle if he flees from his brigade to the commander. Or going to the grand Imam, would also fall under this permission.''
`Umar bin Al-Khattab, may Allah be pleased with him, said about Abu `Ubayd when he was fighting on the bridge in the land of the Persians, because of the many Zoroastrian soldiers, "If he retreated to me then I would be as a troop for him.''
This is how it was reported by Muhammad bin Sirin from `Umar. In the report of Abu `Uthman An-Nahdi from `Umar, he said: When Abu `Ubayd was fighting, `Umar said, "O people! We are your troop.'' Mujahid said that `Umar said, "We are the troop of every Muslim.'' Abdul-Malik bin `Umayr reported from `Umar, "O people! Don't be confused over this Ayah, it was only about the day of Badr, and we are a troop for every Muslim.'' Ibn Abi Hatim recorded that Nafi` questioned Ibn `Umar, "We are people who are not stationary when fighting our enemy, and we may not know where our troop is, be it that of our Imam or our army.''
So he replied, "The troop is Allah's Messenger.'' I said but Allah said,
(when you meet those who disbelieve in the battlefield) to the end of the Ayah. So he said; "This Ayah was about Badr, not before it nor after it.''
Ad-Dahhak commented that Allah's statement,
أَوْ مُتَحَيِّزاً إِلَى فِئَةٍ
(or to retreat to a troop), refers to "Those who retreat to the Messenger of Allah and his Companions (when the Messenger was alive), and those who retreat in the present time to his commander or companions.'' However, if one flees for any other reason than those mentioned here, then it is prohibited and considered a major sin. Al-Bukhari and Muslim recorded that Abu Hurayrah said that the Messenger of Allah said,
«اِجْتَنِبُوا السَّبْعَ الْمُوبِقَات»
("Shun the seven great destructive sins.'')
The people inquired, "O Allah's Messenger! What are they'' He said,
((They are:) Joining others in worship with Allah, magic taking life which Allah has forbidden, except for a just cause (according to Islamic law), consuming Riba, consuming an orphan's wealth, fleeing the battlefield at the time of fighting, and false accusation to chaste women, who never even think of anything touching chastity and are good believers.)
This is why Allah said here,
(he indeed has drawn upon himself...), and returned with,
بِغَضَبٍ مِّنَ اللَّهِ وَمَأْوَاهُ
(wrath from Allah. And his abode...), destination, and dwelling place on the Day of Return,
جَهَنَّمُ وَبِئْسَ الْمَصِيرُ
(is Hell, and worst indeed is that destination!)
Maududi - Sayyid Abul Ala Maududi - Tafhim al-Qur'an
The Qur'an does not forbid an orderly retreat if necessitated by military strategy. It is lawful to retreat if there is a hard pressure of the enemy, and the fighting troops retreat to get re-enforcements or to join another pan of the army in the rear. However, what has been forbidden is a rout in utter defeat, involving disorderly and cowardly flight for safety. Such a retreat is obviously a heinous sin, because it is to save one's own life and it deserves the most painful retreat in Hell. Any one, who runs away in disorder from the battlefield, does so because he loves his own life more than the cause for which he professed to fight. Accordingly the Holy Prophet condemns this emphatically, saying, "There are three sins which make virtue vain-shirk, violation of the rights of parents and flight from the battlefield, when fighting in the Way of Allah." Likewise in another Tradition, he mentions seven sins that are ruinous and destructive for the Life-after-death. One of these is that one should turn one's back to the disbelievers and run away from the conflict between Islam and kufr. Such a rout is condemned because, besides being a cowardly act, it leads to serious consequences: the flight of one soldier may cause the rout of a platoon and that in turn of a regiment or of the whole army. Then it is also possible that the rout of the army might ruin the whole country.