Aslan is advising Peter how to conduct military operations in the forthcoming battle against the forces of evil: where centaurs should be deployed, where scouts should be posted, and so on. Peter doesn’t quite understand why Aslan’s giving him such detailed instructions. Won’t he be there in person to lead Peter and the others? Peter’s shocked when Aslan tells him that he can’t promise that he will be.
Without the big lion leading from the front, Peter doesn’t know how he’ll cope in the face of battle. It all seems such an incredibly scary prospect all of a sudden. No wonder, then, that morale in camp that night is so very poor. Everyone's nervous about the battle and what it may bring. So long as they thought that Aslan would be right there with them in the thick of things, everyone was upbeat. But after Aslan's shocking confession that he can't promise that he'll be present, the mood has changed completely. Now it seems that the good times, having only just begun, are drawing to a close.