The Achdus vanished over a century ago when a large number Ashkenazi Jews went off the derekh. A major factor during World War II is that the hiloni Jews assumed Hitler was only out to get the frum Jews, and were shocked when he turned out not to like good, western, irreligious Jews. In America, the secular Jews actually lobbied the American government not to support acitvities designed to rescue Jews (e.g. bombing the concentration camps and the railroad leading to them). If you try working outside the frum “ghetto”, you’ll discover that most American goyim are quite tolerant and even supportive of frumkeit in matters such as kashrus, Shabbos, clothing, etc. — where as the secular Jews object strongly to accomodation of frumkeit.
Regardless of the halachic definition, for practical purposes a Jew is someone who is Shomer Mitsvos – and defined that way, there is plenty of achdus even between the streimels and the kippah serugahs, and all points in between.