Superman has been a comic-book hero for a long, long time - so long that his tenure as the Man of Steel included World War 2 - but when characters exist for long enough, they tend to attract some awkward questions - in this case, why didn't Superman fight in WWII?
By the time America entered WWII in 1941, Superman was already a household name appearing in comics, newspapers, and on the radio. Each week, the Man of Steel beat up bad guys and saved the day with a wholesome, corn-fed optimism. So, where was he when America went to war? The truth is, he offered to help out, and America turned him away.
A newspaper comic from 1942 titled “The Failure” explains why. This short story shows Clark Kent as he argues with a recruiter and a physician, expressing absolute shock and dismay that he is being rejected by the army. The doctor explains that while, yes, Clark is in perfect health, he is, unfortunately, “blind as a bat” and that when he read the eye chart he “muffed every line”. It is then Clark realizes that he accidentally used his x-ray vision and looked through the wall at the eye chart in the next room! Dejected, Clark leaves, having to break it to Lois that he failed the physical.
Clark’s discouragement didn’t last for long, however. By 1943, Superman was flying out to help American soldiers when newspaper comics began having servicemen and women write in to Superman, asking him to solve small day-to-day problems. Suddenly, Superman had a job to do again! No longer stuck in Metropolis, the Man of Steel was now helping deliver mail and taking over kitchen duty. Still, it's fair to ask why Superman wasn't winning the war single-handed. The truth is that Superman lived in a world where he wasn't needed to win the day. Built into these comics was a sort of patriotism that often bordered on jingoism; the average American soldier was mighty and capable against the enemy, so there was no need for Superman.
The need for comic creators to explain away why a man strong enough to likely end the war on his own wasn't even fighting is fairly obvious; even if Superman ended the fight in the comics, it would have still carried on in the world beyond the page. Having Superman participate in the patriotism of the time in small ways was a smart move for creators, even if they had to make him look like a doofus and “muff” his eye chart to get there. In many ways, it was also the more respectful option - while Marvel heroes like Captain America and the Invaders fought invented enemies to justify why they weren't winning the war on their own, Superman's contribution focused on the people he was able to help; less flashy than the Captain America cover where Steve punches Hitler, but just as true to what made Superman great then and now.