Chuck, I think that your causal claims here run backward. A sort of spontaneous chauvinism, which may go back scores of millenia, doubtlessly helped smooth the way for this slaughter. BUT, Racism did not cause the slaughter, the slaughter (and later the enslavement) caused racism.
Understanding this is of crucial importance to the present, for if racism is seen as an independent cause of oppression *rather than* as an ideology which develops in daily life to make sense of empirical experience, then in fact racism is dehistoricized in that the name of what needs explanation becomes itself an explanation.
Once this is grasped (there is a large literature developing the point; I myself have found Barbara Fields's work most illuminating), then it is possible to see that the ideology of racism may be changed only by FIRST changing the objective oppression of black people. That is why, as has been shown also in practice over and over again for more than a century, that left or working-class activity that does not incorporate *from the beginning* both a struggle against racism AND black people and women in its leadership will, willy nilly, ultimately betray itself.
It is within this restricted compass that I argued a week ago that the most serious *immediate* enemies to working class progress in the United States were those who, however good their attentions, blocked in any way the development of anti-racist struggles within the working class movement itself. (And, given the history of Europeans in the U.S., and particularly of male Europeans, I am increasingly skeptical of even the good faith of those who argue that working class activity can develop in abstraction from the struggles against racism and sexism, and that those struggles can develop without leadership coming, perhaps predominantly, from non-whites and women.
What drove the Weathermen of the 60s insane (and they really were pretty loopy) was that they saw what an impossible barrier to progress racism was, but could not, not really being marxist despite their rod books, imagine the kind of struggle which might overcome that internal barrier.
Carrol