although ere that they had vowed before God that they would never turn their backs [on His message]: and a vow made to God must surely be answered for!

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

And verily they had already sworn unto Allah that they would not turn their backs (to the foe). An oath to Allah must be answered for.

Say: Flight will not profit you, if you flee from death or slaughter, and then you will not be allowed to enjoy yourselves but a little.

And they had already promised Allah before not to turn their backs and flee. And ever is the promise to Allah [that about which one will be] questioned.