WWI is actually one of the most interesting parts of the book. I read this book a long time ago so I'm probably remembering a lot of it wrong, but basically, by WWI, the decline already quite advanced, enough to be a cliche: the Gilded Age mansions and museums of New York were already filled up with art and artifacts bought off of broke British aristocrats.

The causes are complex, and interesting. One of the big challenges for the British aristocracy was that American farming started to destroy the economics of traditional agriculture-oriented landholding in the UK in the late 19th century.

So, the 'bargain' the British aristocracy had sort of explicitly or implicitly made over the centuries with the rest of the British population was that the aristocracy were, you know, the nobles. The knights. We call them 'armigerous' families because the have a _coat of arms_.

The British aristocracy viewed WWI as an opportunity to be like, "Ok, THIS is our time to shine, this is what we've been playing rugby at Rugby for all these years..." The sons of the British aristocracy signed up eagerly, and were put into leadership positions. In the early part of the war, they were still using tactics oriented around personal bravery and charge-leading, so while WWI was horrific for all of Europe with (IIRC) ~10% of the of-age men being killed in the war, the impact on the aristocracy was much worse, with some estimates being around 30% of the young men of the aristocracy dying in the war.

There were far-reaching restructurings of society after WWI, but the aristocracy couldn't come back from the combined loss of a generation of a men, and failure to achieve their gambit: they basically bet it all on "we'll show you, we'll lead the nation through this war" and the public perception was that they completely failed.

So it's not so much that WWI did it, as it was basically done, and WWI was a last gasp.

Like I said though I read this book in...I wanna say 2006? It's been a while.