Slaughterhouse Five

"Later on in life, the Tralfamadorians would advise Billy to concentrate on the happy moments of his life, and to ignore the unhappy ones - to stare only at pretty things as eternity failed to go by. If this sort of selectivity had been possible for Billy, he might have chosen as his happiest moment his sun drenched snooze in the back of the wagon." 249

At this point in the book, Billy and five other Americans have gone back into Dresden two days after the bombing to go back the the slaughterhouse. Billy stays in the wagon once they get there and takes a nap. I thought it was really strange that this moment was described as one of the happiest moments for Billy. At the beginning we are told that this book is an anti-war novel, which is why it was weird to me that the main character's happiest moment is at a time when he is at war. Although this moment does come two days after the bombing, it is still in a time of war. This book could have continued on just fine without mentioning that this was his happiest moment.

Why does Vonnegut tell the reader that this might have been Billy's happiest moment? If this is an anti-war novel why does this moment come during the war?