Captain America Comics #1 was published in March of 1941, back when bashing Nazis was, believe it or not, a pretty controversial subject matter. Since the U.S. wasn't involved in World War II at that point, a lot of people were very upset when they saw the literal captain of America punching the leader of Germany. Some were "America Firsters" who thought the U.S. should stay out of whatever was going on in Europe, and some were plain ol' Nazis who liked what this Hitler guy was about. Soon, the Timely Comics (later renamed Marvel) offices were "inundated" with angry letters and phone calls, a common theme of which was "death to the Jews."
Then Nazis started dropping by the building. New York City Mayor Fiorello LaGuardia had to get involved and personally offer police protection to the Timely staff. Not that all of them needed it. Jack Kirby once answered a call from self-described "real Nazis" who dared him to come down to the lobby for an ass-kicking. Kirby said he'd be right there, but by the time he arrived, the Real Nazis had vanished. Considering that Kirby went on to kill some Nazis in the war, maybe that was a good call on their part.