In The Difference Engine, by Bruce Sterling and William Gibson, the point of departure is that Charles Babbage got the funding to make his proposed Victorian calculator, the Difference Engine. Now, in this world, the USA has collapsed into at least three countries IIRC (USA, Confederacy, Texas). How did the completion of the Difference Engine lead to this happening?
In short, the Difference Engine and the technocratic revolution strengthened the UK to the point that it was able to exert force and influence more effectively during the United States' formative period. It would not have taken much of a thumb on the scales to turn the Civil War into a stalemate and Texas into... well, Texas.
Support for this theory:
- The UK demonstrates much more advanced military powers than in the true timeline - camouflage, scientifically targeted artillery. This is supported in part by Tom Mallory's comments about his time in India.
- The UK is more united than in the real timeline; their charitable intervention during the Irish Potato Famine demonstrates this.
- The UK is led by scientists and technocrats who have learned to apply scientific methods to problems without being hindered by the past; fundamentally, they are revolutionaries (or catastrophists!). In the real timeline, the UK's lack of such flexibility greatly contributed to their slowly ceding the world stage.
Looking at the Wikipedia entry for the book, it would seem I'm not the only one with this theory:
the United States became fragmented, due to interference from a Britain which foresaw the implications of a unified United States on the world stage.