ONE MIGHT HAVE THOUGHT NAIVELY

that the US, with its equally naive multicultural ideology, would have been able to eagerly embrace, socially and economically, newly emancipated blacks, Jews, Catholics, Muslims, Mexicans,  etc., quite readily.

That has emphatically not been the case.

One might have thought, equally naively, that France, for example, after the French Revolution, 'liberty equality fraternity', would be equally open to Protestants, Jews, Muslims, Slavs, etc. 

Although Napoleon gave Jews relative equality at the time of the Empire, this was gradually eroded as the 19th Century unfolded, until one had the spectacle of the Dreyfus Affair in France in the late 19th Century. 

France at the time of WWI was probably as anti Semitic as Germany.