http://www.supermanhomepage.com/comics/comics.php?topic=articles/supes-war

[quote]

"Battle of the Atoms" was originally going to appear in late 1944, but finally appeared in Superman #38 (January-February 1946) and featured a classic battle with Luthor save for the fact that Luthor's new weapon was an "Atomic Bomb". Since the Manhattan project, which gave rise to the first two American nuclear weapons, was in full swing in 1944, the Defense Department wanted nothing tipping off the Germans that America was even considering work on an atomic bomb, not even from a comic book. While the weapon used by Luthor looked nothing like the actual weapon, and was not anywhere near as destructive as the real bomb, government agents came to DC's offices and demanded that the story not be printed until official clearance was given, citing the need for a unified national defense. Obviously, the people at DC were confused, realizing that they must have come up with something more than their normal fantastic story.

Following that, another story, "Crime Paradise", was also censored and delayed. It ultimately appeared in 1946 in Action Comics #101 and told the story of Superman covering an atom bomb test, actually filming it for the Army. It featured a great cover by Wayne Boring and Stan Kaye showing an explosion with the now familiar "mushroom cloud". [/quote]

"Battle of The Atoms" was reprinted in SUPERMAN, V1 #243.

Kevin