Not in the context you've presented, nor would it work in the context you've presented.
By wiping out a million people you're creating martyrs of them regardless of how you wipe out that culture people are going to take offence at the act and seek a way to reak vengeance upon you and your culture, even if they were (until recently) a part of your culture. Instead of killing 1 million to save a 10 million you've killed 1 million, doomed a 100 million more, erased an apparently promising culture and turned who knows how many people from various walks of life all across the world into your enemy, whether they're willing to kill you and yours or not.
All you can do is what the British did in WW2, show an enemy fairness, even kindness and willingness to act and sacrifice not only to protect yourself but the enemies own civilian population and culture. It is extremely difficult for an enemy to villify you if you go out of your way to help and protect those who, according to the enemies propaganda should be slaughtering and raping their way accross the country. this is something the armies of the 'west' seem to have forgotten.