"Empire Forever!"
I misremember the author, but "The Empire Forever" trilogy is a serious Brit-wank for 2.9 books.. According to the author, having the US united would mean that it was content with its holdings in North America, or would be within a decade. It was only mentioned in passing; the USA was not the main part of the tale, but he is right about the USA having no real need to expand any further. Throughout the 20th century, everyone's policy was "don't piss off the USA," not because it could whip anyone, but because it was so big and industrialized that if it did jump into a war, the war would be rapidly decided. And when Venezuela went too far in the early 20th century, it finally got snapped up by the USA, and made a permanent protectorate--this lesson was not lost on the rest of the Americas.
This was settingthe scene for "Empire Forever," and sequels, a constant tale of war and peace through Europe and Asia, with the USA conviently out of the way--though he did use it to good effect in half-neutralizing Britian's command of the sea; when the Royal Navy interdicted war materials to neutrals, their President Bryant insisted that American rights would be backed up by American battleships--so endless American war materials flowed to neutrals--who promptly sold them across the border to France.
The finalle is rather unique, as the British Empire along with the Prussian Empire becomes dominant in Europe, crusing France and driving Russia out of Poland, and keeps its colonies. Yet, at the same time, the USA, with vast hordes of European gold gained by selling so much war materials, is by far the richest nation on Earth--motorcars in the hands of even the poor, aeroplanes carring ordinary people on vacation, atomic power plants, and even computing machines for sale in department stores.
The last scene, of Americans in their space station, looking down on the planet-boud Europeans' "petty squabbles" makes a decent A/H war story into something else, asking the readers to question the value of war as opposed to the value of staying out, and building an economy. As one character said, "The USA decided to get the tecnological benefiots of war, yet omit the war."
(I must say, at least it makes sence that a nation that, through a half-century and more, makes a lot of money selling anything to anyone, would have incentive to improve its technology, so as to keep its products valuable, and defend its shores. And the constant bombing in Europe would slow down any reserch there.)