Interesting scenario. Good points raised by all who posted here.
One thing that I would like to add is this: given the parameters of this what if, I would assume that the militaristic cabal that took over the Japanese govt would lose its steam in the years following the establishment of such an alliance. Thus, the Japanese govt would be more likely be in the Western mold instead of what arose in World War II. Since the 1920's was mentioned, the Japanese govt and military then was more professional and conservative and patterned after the British, based on what I've read.
Of course, I'm operating from hindsight. Feel free to pick holes.