I've always wondered what would have happened to Japan if the US forced them to allow EU-style free movement between the two (or more?) countries while they were drafting their new constitution.

Please note I'm not suggesting the US should have done that, just noting it wouldn't have been that wildly different than the US and Puerto Rico just a few decades earlier.

I'm not a history buff and I'm sure it's way more nuanced than that, just surprised US imperialism didn't swing for the fences like it had done before. Considering the atrocities Japanese-Americans faced during those times I can see why neither party would've been keen on opening up like that, I know.