A wind of sorrow whistles around the King's battle pavilion. It is late and his head is bowed over his papers. He is tired; he has been broken by the recent two battles. The Round Table is now dispersed. He feels that he has failed the service Merlyn trained him for—to conquer Might with Right. He sees now that his whole reign has been based on a premise that is false: that man is decent.
Just as Merlyn foresaw years before, Arthur's attempt to innovate the political system in England has failed. Arthur thinks it is because of the false ideal Merlyn instilled in him—that man is ultimately good. This idea is the opposite of what the bible teaches and exposes Arthur's ultimate humanity.
He thinks now how his whole life has been an attempt to plug a raging flood, only to have it break through in places he has overlooked. The King realizes the Bible is right in saying the heart of man is deceitful over all things. His mind circles back over the same things: Why do men fight?
White's novel is a discordant one: most of the book describes questing, warfare, killing, jousting etc. Yet, it simultaneously preaches pacifism: Arthur's ultimate goal is to stop all forms of violence. He fails in his goal, though, because ultimately man is flawed.
He thinks about the problem of having; how some men have and others do not. About if all men were equal, there would be no need for war. Or perhaps wars happen because of fear, fear of the other, of your neighbor who you do not trust.
Arthur's mind dwells on the possible reasons for war: inequality, xenophobia etc. Yet having so many different reasons for war suggests that perhaps there is no reason for war, that perhaps the reason men fight is no different than the reason ants fight—because they can.
A page enters and Arthur asks his name—he is Tom of Warwick. Arthur asks him not to fight in the battle tomorrow, but to ride home instead and take Arthur's Idea with him so that it will last. Arthur tells him the story of the table's creation and of his attempt to bring justice to England. The page says he will leave and tell everyone of this ancient idea. Arthur then knights the boy and tells him to rise: Sir Thomas of Warwick.
Sir Thomas of Warwick is in fact Sir Thomas Malory of Warwickshire—the believed author of "Le Morte d'Arthur" on which T.H. White's book is based. By introducing Malory here as a young page, told the story by Arthur himself, White creates a way for Arthur to pass on his ideals even as they die away—through writing, a legacy that will pass down to White's own novel. In this way White subverts the tradition of the Arthurian myth (which has always glorified war and fantasies of English nationalism) to rather be one that argues against both of those ideas and instead focuses on pacifism and a desire to eliminate nationalism.
Arthur sleeps and dreams of Merlyn. When he wakes he begins to remember: Lyo-Lyok and the birds migrating; the belligerent ants; the badger. He suddenly sees the problem plain before him. War is fought about nothing, literally nothing: it is based upon imaginary lines on a map. Nations are simply that and their feuds are about lines that do not exist. If only the imaginary lines on the earth's surface could be unimagined.
Throughout his lifetime, Arthur barely thinks about the times he was changed into animals by Merlyn. However here, on his deathbed, Arthur remembers all of the animal friends he had made and what they had taught him. He suddenly understands that if man were able to dismantle all the divisions between nations, there would be no reason for war.
The King feels refreshed and clear-headed. He is ready to reform the table and bring his new idea. But it is too late for him. It is his destiny now to die; Guenever's to take the veil; and Mordred to be slain. The cannons begin to thunder and the King of England draws himself up to meet the future with a peaceful heart.
Although Arthur suddenly understands why men commit violence, he will never get to act upon it because he will die this day in battle—he will die in violence. The story ends with him at peace for having realized the answer to the questions he has been asking, and yet failing to put those answers into any sort of practice. The ideal Arthur realizes once again seems to be out of reach of men in the real world, just as the grail was, though there is also a hopeful cast to Arthur's thoughts and to the ending of the book, as if perhaps both Arthur and White hope that their message and ideas might still be taken up at some point in the future.