Story where America is conquered by all other nations of Earth

I remember a science fiction story where America is conquered because every other nation on Earth declared war on it, and America was unwilling to actually fight the entire world, so it surrendered.

As I recall there was no actual precipitating event to the declaration, and the first bunch of nations to declare war were very small and weak ones, like perhaps Lichtenstein or Liberia.

There were scenes showing the UN occupying forces on the streets of an American city.

I have a hunch this may have been a Fred Pohl story. I have checked The Early Pohl and Platinum Pohl but have not found it. It may have been in the same book as "Poor Superman" and the other postwar American stories, but it was definitely not in the same worldline. I’m guessing I probably read it during the 1990s, but it is most likely published earlier than 1975, perhaps as early as the 50s.

Although I’m remembering it as science-fiction, there really were very few if any actual SF elements in it.

EDIT: Duh, "Poor Superman" was Leiber, not Pohl. So it may have been Leiber. Still can’t find it.