Tafsir of Al-Ahzab 33:15

(And they had already promised Allah before not to turn their backs). These are the group who sought permission [to leave], and they are the Banu Harithah according to the majority. It is said: they are the Banu Salimah who had been cowardly on the day of Uhud, then repented and promised on that day, before the Day of the Trench, not to flee.

According to Ibn Abbas, they are a people who promised in Makkah on the night of al-Aqabah that they would protect him—may Allah bless him and grant him peace—from that which they protect their own selves from. It is also said: they are people who were absent from the battle of Badr and grieved over what they had missed of the honor granted to the people of Badr, so they said, "If Allah the Exalted lets us witness a battle, we shall surely fight."

The verb (promised) is treated as an oath, and that is why it is followed by His saying: (not to turn their backs). It is brought in the third person for the sake of the meaning; had it been brought as they had uttered it, the structure would have been "not to turn your backs."

"Turning the backs" is a metonym for fleeing and defeat, for the one who flees turns his back upon the one from whom he flees.

(And the promise of Allah shall be inquired about); that is, the fulfillment of it will be questioned, and one will be recompensed for it, and that will be on the Day of Resurrection. The use of the past tense, according to Majma’ al-Bayan, denotes the certainty of its occurrence. It is also said: it means it was inquired about with Allah the Exalted regarding the fulfillment of it, or it remains a binding demand until it is fulfilled.