Well after all, racial reasons behind it. Both China and Japan are considered non-white, thus non-aryan, let alone anglophone. This makes those two nation 3rd ranked (1. Anglos, 2. other 'whites', next being all other races ordered by skin color, with E Asians being basically white in skin tone, still considered yellow?!). A non-white, nation surpassing whites is a fundamental threat of existence to the white supremacists (Even if not actively thinking this way, almost all whites have a (sub)concious superiority complex).
Therefore I don't think they want China, even as a puppet become stronger than themselves. Since Japan they became alot more paranoia about non-whites surpassing them.
Look at the Japan-bashing era of the 1980s, when the US thought it was going to be left in the dust by Japan.
Japan was a democracy and the US had a far larger population. So realistically, the odds of the US being left behind were pretty low AND even if it was, was the resulting situation really going to be that bad?
Yet it quickly turned into a racial attacks on Japan and anything that looked Japanese.
More recently, look at the race-baiting involved with Obama and Trump.
So it's only going to get far worse in the coming years as China continues to grow faster and that is converted into economic, technological and military influence