These Fair Shores: The Commonwealth of New England

The only states to successfully secede were Caribbean islands with mixed Anglo-Hispanic populations. Louisiana was the only continental state to make a go of it, and their attempt was crippled by the storm damage that flooded New Orleans. In addition, the "Provisional Government" the Empire tried to prop up in the Northeast lacked any legitimacy from the get-go, and this combined with the indiscriminate bombing campaigns served to unify the rest of the country against the British threat. Finally, Hogan's purge of the rogue CPU officials and the Empire being forced to look inward after Quincy's attempted autocoup ensured that neither side wanted the war to continue, so any remaining separatists were hung out to dry.
the Empire tried to prop up in the Northeast lacked any legitimacy from the get-go,
If I'm not mistaken. A funny tidbit about it is that this provisional government tried to privatize everything so much that they end up being heated by the people it was supposed to govern, and the British also end up hating it because, again, they tried to privatize everything. Privatization is iirc far less common in this TL, so government control is far more widespread as a result. Another funny tidbit is the """government""" in exile located in Toronto, it's as far as I understand it, the John Birch Society, it was created after the 17 revolution and the funniest part about it is that they were not even invited to the provisional government. So they are just very angry racists stuck in Toronto while claiming to be the legitimacy government and the British are just like "Uh huh, ok" proceeds to make own government.
A country doesn't need foreign intervention to disintegrate into smaller countries.
Why would the US disintegrate? Pre-Hogan, the DSU was widely acknowledged as corrupt and authoritarian, but the state autarchic system was keeping millions of coal miners and other workers from the ravages of the free market. Not only this, but many groups, like Natives, bureaucrats, and even many average workers genuinely believed in the Union's mission. The US only exploded into conflict after years of Hogan's privatization efforts, and while the Caribbean remains independent, the American mainland is now firmly under his control.
Balochistan; Deparments of Balochistan