Reality Subtext: White wrote the majority of the series, including The Book of Merlyn, during the darkest days of World War II. It definitely shows, if you're looking for it. The scene with the ants in The Book of Merlyn is an explicit parallel to totalitarian regimes and Nazism in particular. And Mordred's faction in The Candle in the Wind are described in ways intended to compare them to Nazis. Plus there's Merlin's angry lecture to Sir Kay in The Queen of Air and Darkness, although that's less subtext than outright text.