It's the Great Pumpkin, Charlie Brown (1966)

So...It has the Red baron Snoopy WWI fantasy...and that is what you remember. That is the most awesome part of any Charlie Brown movie. And...beyond that you have a kid making up a story and believing it that would, almost 30 years later, be spoofed in an episode of the Simpsons. So it has staying power. But really, it's a childhood classic that you love first because it is wonderful and touching, then later because it's nostalgic, and then later because it is a generational tradition. In other words it stands the test of time like the Godfather and so many other pure classics.