Ahem...
American Indian tribes continually warred against their neighbors, took slaves and interbred with them. That's intermixing, of genes anyway.
You are right. I was unclear at first. As I made clear above, I excluded the 'taking of slaves' scenario simply because we do not find intermixed communities in the fossil record. If Neanderthal women were taken slaves (and who knows why a Cro-Magnon caveman would want to take an unintelligible butt ugly musclebound Neanderthal female home, but let's leave that aside) then that would be an intermixed community. We don't find them in the fossil record. It doesn't definitively mean that they didn't exist, and you're welcome to fantasize them all day till the butt ugly cows come home, but we haven't found evidence of them if they did exist.