The prequel to the Tripods trilogy is When The Tripods Came. It is a very well written story of how the Tripods conquer the Earth.
Not through violence, or at least not entirely. The initial attack is repelled successfully by the armed forces of the various nations that are invaded. But the Tripods are subtle.
They begin using subliminal persuasion on broadcast TV programs, and other methods of mind control. This leads to the young being converted into ‘Trippies’ and they begin the illicit distribution of caps.
Slowly but surely the world is taken over, with some fighting here and there.
The story is told through the eyes of Laurie whose family is involved in the initial Tripod attack in England.
They flee to Switzerland eventually where the Tripods are resisted most strongly. Eventually even that country falls and they flee to a remote mountainous part of Switzerland. (The White Mountains) where they start the resistance.
This 4 book series is one of my favorite juvenile SF readings.
One interesting thing that I discovered while looking for the first time in a while at my copy of this prequel. The blurb writer mentions that there was a BBC series called The Tripods and that it also played in the US.
I have never seen or heard of this series. Well, yet another thing I will have to google and look for on Amazon or eBay.