I vote for Indo-European-speaking invaders. They didn’t expand from India to the Iberian peninsula by being nice.
May have been them but could have been one of the imperial powers from just over the threshold of the age of literacy — the Elamites (who had a writing system, but it can’t be read, so far) would be a guess. The Indo-Europeans entered Europe and the Middle East (and India, for that matter) in a series of waves, which at least in part coincide with the global climate cycles.