What is the theme in When the Tripods Came by John Christopher?

The importance of free thought is the narrative's central theme, anchoring and defining both its action and the personal transformations of its characters. On one level, exploration of this theme applies on a book-specific, literal level. Everything the Tripods do is an assault on free thought, while everything that Laurie and the other principal characters (Pa, Andy, Martha, Ilse, and the others) do is a defense of free thought. On another, more metaphorical level, the narrative's exploration of this theme is a condemnation of such assaults in general.