You can get full USA control on February 1, 2023. Are these other strategies even faster? That's wild.
The point isn't to get the USA faster.
Before getting tired of it and ending my game, I ended up with USA, most of the EU, Russia, and China. I was over my management cap, but had enough influence gain to keep up. By keeping my own popularity up in the other three, the AIs left them alone until I was done taking undefended EU provinces. All my councilors were persuasion classes, except for a spy I picked up later. Everyone being able to campaign for popularity and take control of neutral areas, and do them well, helps a lot.
I think I had 13 persuasion on my agent who started in on China, with about 33% odds per attempt with 32 influence spent, and about 66% of the population loving me. It took a lot of 25% chance public campaigns with $640 spent, but I was swimming in money by that point.
By the end, I think I only had about 55 boost, after spending 10 each on a pair of Mars bases, that would have taken a year to complete. My moon base gave me enough resources for both, other than the titanium. I think that reduced the Boost costs to only what the titanium purchase cost was, but there might have been a small extra cost.
I didn't build any ships, thinking a 60 power ship useless against the 7k alien power. I should have built one to see if they do anything interesting against other humans. I expect they can shoot down stations, but being able to lock down all of the orbits near Earth would give too much power to whoever gets there first.
How *exactly* do Federations and Unifications work? China starts with a claim on Taiwan which means you can unify the two. But there's also a project called "Liberating Mainland China" that gives Taiwan claims on, well, mainland China. What's the benefit? If you win a war against a country, you install yourself in all their control points, so there's no reason you can't just unify from the PRC side once you've won. I mean, I like the RP potential of putting ROC in the driver's seat, but "Liberating Mainland China" costs 30k research which seems like a tough pill to swallow for what's basically just a name swap -- unless there's some benefit I'm not aware of, like maybe the other party inherits the tech levels of the leader or something.
If you win a war and have a claim, it gets annexed outright.
China is hard to break into when it's neutral and you have high popularity. If someone else takes it, you're not taking it from them with spies under normal circumstances. I assume that research is so you can take the much easier Taiwan, ally them to superpowers who can beat China, and then declare a war from Taiwan and pull the superpowers in to win the war.
Federations let you diplo annex if you control both governments. Until then, they share Funding and Boost, apparently giving an equal share to every province in the federation. It seems more annoying than it does useful.