Unification of Countries Guide
Assuming Country A is the nation that is the primary nation, and Country B is the nation that is going to be unified into Country A, then there are two paths.
Peaceful Unification
Country A must have a claim on the capital region of country B to unify peacefully. Through war you will get any regions country A has a claim over.
Control both nations, and control ALL the Control Points in Country B. Then make both nations join the same Federation, afterwards you can have both nations unify. There is a cooldown period between these actions which can take a year or so to process. BE WARNED that all CPs in Country A are inherited when the new nation is absorbed, so if any CPs are owned by another faction, they will keep them in the new nation. This can make it harder to remove them later.
Forced Unification
Control Country A at minimum, control over Country B is optional but preferred. Have Country A conquer the capital of Country B, this will immediately unify Country B with Country A (this can happen in game if Russia conquers Ukraine). This is faster than peaceful unification, but it will likely cause some economic damage due to the war
There is a pseudo edge case where a nation gets claims on another but it cannot unify. From my understanding the nation that is wanting to unify must have a larger GDP than the nation it wants to absorb.
So if Country A has a GDP of 50b and Country B has a GDP of 200b, then it cannot unify. You can get around this by having Country B cede territory to Country A, which will give it all of its regions except its capital.
I believe so, I’d have to watch more closely the next time I unify via war. But I think when I was unifying South America I had an army of 4.2 and was absorbing nations with 3.8 and lower and by the time I unified the continent it was down to 4.0. I will admit this was with both peace and war, so this is purely anecdotal.