The Once and Future King Book 4, Chapter 9

Six months later, Guenever and Lancelot are at Joyous Gard, which is surrounded by Arthur and his troops. Lancelot tries to explain to Guenever what happened to Gareth and Gaheris: he was surrounded, and had to cut his way through the crowd to rescue her. He could not avoid killing them. Lancelot feels that the entire war is his fault: he thinks that Arthur would have forgiven him, but now Gawaine never will. Lancelot doesn't know what to do: he doesn't want to fight, but putting it off is causing the deaths of many lower soldiers. Suddenly it occurs to Guenever that they could appeal to the Pope. He could threaten to excommunicate them unless they stopped fighting, and Lancelot and Guenever could accept whatever terms he imposed on them, and peace would come. Mordred and Gawaine would have no choice but to listen to the Pope.