Postal history from any branch of the armed forces is fine, postmarked sometime in the year 1941. There have been some great examples posted on the forum in the past, let's see what everyone can come up with.
To get the thread rolling I'll repost this:
BB-39 U.S.S. Arizona was lost during the Japanese attack on Pearl Harbor, December 7th, 1941, nearly 1,200 of her crew were killed on that morning almost 72 years ago.