I just bookmarked this rec last night to read so I'm not sure how exactly it goes but the summary seems to fit your request. Hope this helps!

Title: We Know Each Other As We Always Were
Author: mickeym

http://mickeym.livejournal.com/1029663.html

Summary: in 1941, while the world is at war, Sam Winchester falls in love with his brother. They're young, they're in love, and in spite of the hardships of life around them, the world is a pretty good one for them. Until Dec. 7th, 1941, when Japan launches an air attack on Pearl Harbor, sending the US to war against Japan. Dean Winchester feels he needs to join the Army; needs to help fight the good fight and help save lives. He promises he'll return, but can he keep that promise?

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Thank you! This is so exactly what I was looking for. I'm excited to give it a shot :)

http://kajikia.livejournal.com/10295.html - World War II AU - Sam/Dean, but they're both soldiers, so it's not quite what you wanted.

http://mickeym.livejournal.com/1029663.html - Here's the one I was looking for. Sam/Dean AU, where they're raised by Bobby, and Dean goes off to war.

In 1941, while the world is at war, Sam Winchester falls in love with his brother. They're young, they're in love, and in spite of the hardships of life around them, the world is a pretty good one for them. Until Dec. 7th, 1941, when Japan launches an air attack on Pearl Harbor, sending the US to war against Japan. Dean Winchester feels he needs to join the Army; needs to help fight the good fight and help save lives. He promises he'll return, but can he keep that promise?

Both perfect :) I'm sure them both being soldiers will be just as satisfying. Thanks for the recs!

Bonjour, mon ami!
I am not completely sure as to whether or not this rec is up to your instructions, but it is a fantastic read from an awesome author.
And I apologize if anything is wrong with the link, I am not used to posting. Just lurking.
Enjoy!

http://morrezela.livejournal.com/16379.html?thread=306939#t306939

Thank you! I do already know and love this little ficlet, but it is exactly the kind of thing I'm looking for. :)

Bring the Rain: http://sinestrated.livejournal.com/52944.html