The central theme in T. H. White's The Once and Future King concerns the conflict between Might and Right. Prior to King Arthur's time, as young Wart was once told, "Power of the body decides everything in the end, and only Might is Right" (Part 1, Ch. 5). In other words, prior to King Arthur's time, there was a dominant belief that war was the natural and Right way to solve conflicts. Yet Merlyn tries to teach Arthur that war is unnatural, that there is nothing Right about Might, and that what is truly Right can be preserved through other means than war. All throughout the book, loyalty is also demonstrated through Arthur's loyalty towards Merlyn's teachings and loyalty towards Right.
One example can be seen in Part 4 of the book. By the end of the book, a war has broken out in France between Gawaine and Lancelot, and Arthur joins Gawaine's side, saying it's his duty as king to pursue justice. However, being involved in a war gives him the moment to reflect, "The fantastic thing about war was that it was fought about nothing--literally nothing," which was also his moment to be loyal to Merlyn's teachings. Though he did feel the need to continue fighting the war and though he did die in battle that day, he continued to see the truth in Merlyn's teachings and be loyal to his teachings though the world forced him into actions using Might that he knew was not Right.