Ouch. Surprised it went that far. Was it just the British going all out in making an example of the French?
Basically, France after the "Great Global War" was very unstable and prone to a lot of factionalism. The late 40s, 50s, and early 60s, where just chaos and turmoil as every time the ITL tries to prop up a regime it gets torn down almost instantly. So the British tried this fancy new bomb to 'put the french in their place'.
A war that lasts seven months and basically tears the CUA apart must have been a doozy. I suspect that some internal problems came to the fore during the conflict and gave the ITL a bit of help?
The CUA isn't very capable to wage a war on three separate fronts, Rupertsland to the north, New England to the east, and Louisiana and Columbian/Georgian rebels to the south. It also doesn't help that the CUA is outdated, both governmental and technological wise in the 80s, even with the volunteers and support they received it was just stuck in the 40s and couldn't handle the fact of Modern Warfare.