Translation:

15. And yet they had already covenanted with Allah not to turn their backs and a covenant with Allah must (surely) be answered for.

Notes (Tafseer)

3686. Apparently, after the battle of Uhud, certain men who had then shown cowardice were forgiven on undertaking that they would behave better next time. A solemn promise made to the Messenger of Allah is a promise to Allah, and it cannot be broken with impunity.