Childhood's End was one of the original stories to explore the emotional aftermath of a first contact with superior extraterrestrials. It helped elevate the genre into the realm of literature and it spawned a wealth of works along the same line, such as Independence Day. The book is not a story of warfare, but rather the dynamics of an alien-human interaction. One could interpret it as a commentary on colonialism, but I think the interstellar contact theme is valid in its own right—Clarke's vivid descriptions of plausible alien starships and lifeforms really open one's mind to the possibilities of the cosmos.
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Clarke creates an interesting future history in Childhood's End, but the way he reveals it saps some of the excitement away. "Most people had two homes, in widely separated parts of the world. Now that the polar regions had been opened up, a considerable fraction of the human race oscillated from Arctic to Antarctic at six-monthly intervals..." It's difficult to care about the world he imagined when he describes it in such a detached, encyclopedic way. I'm sure it would have been more engaging were it revealed more through characters.
Another weakness of the book is the Overmind and what it does to the children of the last generation. The Overmind is a vague, psychic, seemingly omnipresent force whose physical nature is not explained. It transforms the last generation of humans into drones with no attachment to their former lives, and no feelings that can be related to. These quasi-characters can be contrasted with the Overlords, the biological beings who actually visit Earth. The Overlords are not human, but they have understandable personalities; it is the Overlords—and Clarke's description of their society—rather than the more powerful Overmind, that truly captivate the reader's imagination.