the whole thing is a moot point- either they have nukes like us, in which case they can drop them from orbit at their leisure, or they don't, in which case we can use ours in a show of force to convince them to leave/surrender.
In the meantime, do we treat any prisoners with respect? Absolutely. Ill trreatment of another group, no matter what the method- only breeds more hatred and conflict. The moral measurement of all civilisation is not how it treats its friends, but how it treats its enemies. Be barbaric to a hostile force and you're no better than the worst xenophobic stereotype you think them to be.
You don't know from the outset that they have no concept of morality, and even if they don't- you're best bet to avoiding future conflict is an attempt to introduce one, or at the very least foster some sort of understanding between the two sides.
I'm actually a bit shattered by how quickly most of you are to jump down to the level you think the enemy is at. Extremely disappointing.