The true horror of war is shown through the destabilizing effect it has on Stanhope in Journey's End.
Prior to the war, Stanhope had considerable talents. He was a gifted student and athlete in college, and described as a "natural leader." His gifts are ones that are valued in war. Physically, Stanhope is described as a "child" at heart and looking like a "boy." Yet, what Stanhope experiences moves him far away from such innocence.
Sherriff shows that three years on the front have fundamentally changed Stanhope. He is a hardened and cynical man. He is afraid of going back home, even for a small period of time, because of his addiction to drinking. He openly doubts his sanity. The strength and focus that was a part of his character before the war have become absent after it. This is where Sherriff shows the true horror of war. It is shown as a transformative process where individuals change from what they used to be into a gutted shell representing what they are.
Stanhope's reaction to Raleigh highlights this. Stanhope is both scared and angry that Raleigh has joined his unit because of the truth that has emerged. The person that Stanhope once was, the hero that Raleigh worshipped, is no longer there. War has taken it away. Sherriff suggests that this is its true terror.