People really over sell the spread of democracy in these places. By US design, Japan was a one party state until after the Cold War, with ~six years of opposition party rule total. Similar happened in South Korea, who considers it's first free election in ~2000, and even then one of their few elections saw intelligence agencies ensure the win of their former dictator's daughter. And Germany had a history of democracy without US influence.
Coming out of the 1st and then 2nd World wars the US seemed to be the voice of reason and alongside Britain (and possibly France) had a strong desire to make the world a better place and not repeat the mistakes of the past - which were all around to see.
But I feel after the 1960's the US has slowly lost that ideal, and the visionary leaders (Woodrow Wilson, Roosevelt or Eisenhower) are no longer anywhere to be found (whether the electorate don't tolerate or desire them I'm not sure).
I think you've got an overly rosy view of pre-1960's history. Post WWI saw all three of those powers continue their colonialist land grabs, and post WWII saw all three commit many atrocities in the effort to check Russian advances.
Both the Japanese and Korean antidemocratic moves took place in that pre 1960 era for example.
But my point is their intention is to exploit the weakness of the democracy they enforce on the target country. And that is assuming this is a situation where they are exporting democracy.
There are also the opposite situations where they declare democratic outcomes invalid because they don't bow down to US interests. They happily support dictators in that case.
I am the first one to be vocal of US imperialism, but you can't deny that it is US influence that caused several modern countries to be democratic.