I first heard this joke when I was at a dinner with a bunch of other Iraq veteran's. Someone said the joke and we all cracked up laughing. So it may be a joke that's only funny to veterans. But, during dinner, there were several Vietnam veterans there too, and one of the Vietnam vets said that he had just used the joke last week--about the Vietnam war--and that the joke has been around for years.
So it's a timeless joke that has been around through the ages. Vets will understand the joke, because we all know that one vet who's every reply is: "You weren't there....."