Musical-theater historian Bordman should have quit while he was ahead (with his impressively researched American Musical Theatre reference)--because last year's Jerome Kern was tedious, and this short new book is a two-part disappointment. The first, longer part (nearly the whole book) has Bordman tracing operetta in pre-WW II America--from US companies of Pinafore (""the real beginning of the American musical theatre as we know it to this day""); to the work of 1890s Americans (De Koven, Herbert, Sousa); to the influx of Viennese imports (The Merry Widow especially); to the post-WW I years when Friml and Romberg and Kern carried on the operetta style while revue/vaudeville elements came to dominate Broadway. All of these influences, however, are treated in better perspective in the many more widely focused studies of American musical theater: Bordman's bland, scholarly heaping-up of detail hardly supports his argument for the present-day viability of most old operettas; nor does he add much to the familiar view of the Broadway musical's overall history, with its much-annotated interplay between European and native sensibilities. And Bordman's skewed, crusader-like approach--finding as much evidence of ""operetta"" influence as he can, season by season--leads him seriously astray in the book's sketchy, disorganized last pages: here Bordman, much too preoccupied with that ""operetta"" label, pins it on Oklahoma! and any other recent ""musical play"" that's not clearly musical comedy (even unlikely candidates like Cabaret). All this labeling and line-drawing, of course, hardly illuminates the works in question; and it misleadingly de-emphasizes the non-operetta influences which can be found in such ""operettas"" as South Pacific, West Side Story, and Sweeney Todd. So, though a few passionate students and buffs will be grateful for Bordman's research here (especially the pre-WW I material), most will find enough on operetta in the broader histories; and even fans of Herbert and Friml may be confused and skeptical about Bordman's underdeveloped application of the ""operetta"" tag to post-WW II musicals.