Frankly, I think it started exactly as a tribal thing, a sure-fire way of being able to tell whether or not a guy was Jewish or not (I guess with women it didn't matter?) ... that, of course, wound up biting them in the ass in WW2. Note that the practice of non-Jewish circumcision didn't become popular in North America until AFTER WW2. I do believe it was sold to new mothers as something that should be done to boys for "health reasons" exactly for to hide who is Jew and who is not, so that "drop yer pants" can't work the next time they become unpopular.