Anyone who thinks outcomes in Japan after WWII were a product of "US imperialism" is a moron.

Except that the survivor's guilt felt by the main character is not about "US imperialism" (in fact the film has nothing particularly bad to say about the US at all), but rather about his failure first to do his duty and die as a kamikaze pilot, then his cowardice as a gunner that might have been able to protect fellow soldiers in the first Godzilla attack but who doesn't pull the trigger.

He does not have guilt for surviving the atomic bomb in Hiroshima or Nagasaki, or for surviving air raids in Tokyo, as he wasn't there for either of those things.

To the extent that the film talks about the US at all, it notes that the US would help them fight the monster except that moving Navy ships to Japan would risk starting a war with the USSR, so the US instead supplies them with some resources and allows Japan to recommission some of their own warships which were otherwise scheduled for destruction after WWII.