ironsheikh wrote:

but doesn't the Koran say not to attack retreating, surrendered and defenseless enemies?

No, it doesn't

Quran 47:35: wrote:

And be not slack so as to cry for peace and you have the upper hand, and Allah is with you, and He will not bring your deeds to naught.


Quran 47:35 - Literal wrote:

So do not weaken/enfeeble and call to the peace and (while) you are the defeating , and Allah (is) with you, and He will never/not reduce your deeds.