When the Tripods Came is a prequel to the YA sci-fi trilogy known as the Tripods Trilogy. In the original trilogy, an alien master race rules the earth around the year 2100. The aliens are never seen and travel the world in giant tripods with prehensile legs (I often think of the Tripods when I see water towers in small towns). The aliens use mind control techniques to control the human population which lives in a low tech feudal type society. Every year young people are brought to the Tripods to be "capped" - a process that involves having a metallic cap attached to the skull that facilitates the control of humanity.
The original series was published in 1967 and 1968. The prequel was published in 1988. The prequel tells how humanity first encountered the Tripods when the Tripods landed on earth and seemed bent on destruction. The Tripods were quickly defeated militarily so the aliens pulled back and began using cartoon shows and pop music as a cover to deliver mind controlling messages. Soon enough, there are fights amongst those that have been mesmerized and everyone else. The mesmerized people attempt to cap everyone and the stage is set for the world that exists in the Tripod Trilogy, including the placid villages of those that are capped and the remote locations of those that continue to resist.
Comic Book Guy
The author, John Christopher (a psuedonym for Christopher Samuel Youd) says in the preface that he wanted to clarify how the world came to be as it was when the Tripod Trilogy began because so many sci-fi fans (I imagine them as British versions of the Comic Book Guy from the Simpsons) were critical of the technology that is described in the book and if this would have been enough to have overcome modern human technology. They were not considering that the level of technology in human history is rapidly advancing and even accelerating, so Christopher felt he had to justify it in some way.