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After Sherston publicly declares that the war is being deliberately prolonged, the authorities avoid a court-martial by ruling him shell-shocked and sending him to Slateford (Craiglockhart) War Hospital. Once the act has been recast as illness, his dissent needs no answer: the institution can treat his conscience rather than engage with it.
The psychiatrist Rivers never tells Sherston his protest was wrong. He listens, asks questions and lets the argument run its course. Sherston comes to trust him more than anyone he has met, and chooses on his own to go back to his men. The persuasion works because it respects his autonomy, yet it still produces the result the institution wanted, which leaves open whether this was healing or a gentler form of control.
Sherston goes back to the front because staying safe while his men die feels like a worse betrayal than fighting a war he condemns. His loyalty to specific people in danger outweighs his opposition to the system that put them there. For him, returning to fight is the more honest way to carry the protest, not a retreat from it.
Sassoon writes as a middle-aged narrator looking back at his impulsive younger self through the thin disguise of Sherston, with irony and some embarrassment. The fictional mask lets him judge himself more honestly than a straight memoir would. The same device also lets him soften, reorder and leave things out, so the book gains candour and loses literal accuracy at once.