The question repeatedly posed about the U.S. Left is framed in negative terms: why has the Left failed to take hold here? Behind this question lies a view--shared by U.S. Leftists themselves--of the European experience as a ""model"" from which American history somehow deviates. Cantor's nominal answer, summarized by the title, is really no answer at all. That the American Left has been divided is inarguable, and Cantor's survey of the convoluted history of groups and groupscules--from the militant IWW to the innumerable Trotskyist sects of the Sixties, from the early ""culture radicals"" (people like Upton Sinclair, Jack London, Max Eastman, and John Reed) to the Black Panthers--adequately plots their self-destructive paths and fratricidal activities. But one could draw a similar map--albeit on a much larger scale--of the European Left's divisions. Earlier, however, Cantor tries a different approach: utilizing--somewhat loosely--Gramsci's notion of ""cultural hegemony"" (much in vogue lately), he advances the thesis that the U.S. working class never developed alternative values or institutions which would seriously challenge those of the dominant bourgeois culture. But this line of inquiry--with its focus not on radical groups but on the social basis for radicalism--is abandoned for the safer topographic narrative. As a short introduction to this material, Cantor's efforts are acceptable, but readers interested in an analytic treatment would do better with James Weinstein's Ambiguous Legacy: The Left in American Politics (1975).