The Dragon that Swallowed the Sun achievement, any requirement other than not being in a faction?

[Doctor Wuhan]

I am achievement hunting and have had several instances of doing one thing wrong to invalidate the achievement. I have beaten Japan back and they have sued for peace. China has joined the allies and I need to strike early while Germany and Italy are at their strongest and the allies at their weakest. If I accept Japan’s peace offer and then return for them, am I invalidating the achievement?

[KriegerKR]

I doubt it, the achievement says Break free from Japan and annex them without being in a faction.

So you could peace them out and war them later as long as you aren't in a faction I'd guess.