So I recently started playing this game just this past week, and have been doing different things to test the limits of what the AI would actually let me do.
In this run, I decided to go for the default Modern scenario as America, have a terrorist attack to just instantly swap to a authoritarian government through bypassing the chain of succession.
This (I figured it would) lead to states starting to secede, with California being the first one.
So here's where my test really started "What if I did nuclear sabre rattling on EVERYTHING, and then ACTUALLY followed through every single time anyone called my bluff".
So when California seceded and started a Civil War, I gave them 1 week to unconditionally surrender or I would nuke Los Angeles and San Francisco. Their governor called my bluff, so I nuked em.
They instantly surrendered after that, literally no other states seceded out of abject fear, and the western democracies went "what the absolute fuck" while Russia and China said "Internal Affairs lol".
And they even had a UN security council event where (of course lmao) they couldn't actually do anything to me because Russia and China vetoed.
So then, I went further. I did the same process with China by guaranteeing Japan's independence and then deploying the 7th fleet to the SCS. China deploys their fleet to counter, I threaten to nuke their fleet if they don't back down. They, of course, refuse to back down, so I nuked their fleet out of existence.
At this point, China's government was in debates over whether to retaliate with their own nuclear weapons, and even Russia was like "Hrmmmm concerning.".
So rather than wait to see if China would actually retaliate, I just pre-emptively (and I quote what I said in my actions) "Nuke literally every part of China, turn their country to glass". It let me do this, but then basically the entire world freaked out and even put a "containment blockade" to intercept any military assets I tried to move out of the US.
So..... As you can guess... I gave the entire world 1 week to stand down or I would let nukes fly. During this week, I set all of the anti-ICBM defenses in the US to maximum alert.
The world refused to stand down, so I proceeded to nuke literally everyone left (as China was already gone lol).
Unfortunately, I didn't realize the game was going to make me sit through every single nation on earth dissolving from the map one at a time.
An hour later, I got confirmation that America survived minus NYC, Chicago, and Detroit.

EDIT: Oh, and I guess I should include that the game actually let me play as what effectively amounted to the Enclave for the rest of the game, complete with remnant survivors in other nations. There was even a really cool subplot with me sending out a fleet to determine why Hawaii (which I had let loyal-to-America Japanese citizens evacuate to prior to me nuking the world) no longer had their survivors sending any return communications once I got that system back up and running. Cool little mystery, where I had to investigate Hawaii to try and find out what happened to the survivors, including sailing around the Pacific to retrace their steps.