What are the motifs in Childhood's End by Arthur C. Clarke?

Enslavement versus free will is a recurring idea. This is the cause for which the Freedom League is fighting. They feel that even though the Overlords have made improvements upon the world, it is not their place to do so. Throughout the story, references are made about the way in which the Overlords are ruling over the humans. At the end of Chapter 2, they are "shepherding mankind" (pg. 24). The Overlords are also referred to as "interlopers" by the extremist group that kidnapped Stormgren (Chapter 3, pg. 41). In his farewell letter to Maia, Jan feels that the Overlords have been "keeping us in the nursery" (Chapter 12, pg. 119).