I have an alternative theory, which is based on the observation that Siegel published kind of a lot of Superman Goes To War stories prior to the US' entry into WWII, but after the war had already started in Europe. Moreover, the wars in Superman comics in those days never seem to involve the US at this stage, Lois and Clark go abroad to fictional countries to cover them.
My theory is that it was Jerry and Joe using their strip to comment on global issues. War wasn't some hypothetical possibility, it was something happening Right Now as Jerry wrote it; it was a Really Big Deal. And of all the things you can say about Jerry Siegel, you cannot say he didn't try to be topical.
Also in one of the wars from the newspaper strip in probably either 1939 or 1940, one of the leaders looks exactly like Adolf Hitler. His name was Amork, I believe, and the story re-used the ending of Action # 2 where Superman forced the two leaders to fight in front of all their men to showcase how ridiculous war is.