With that in mind, here's the gist of what I did:
1. Rush communism via civil war. You need it by summer 1936 or everything else isn't going to work.
2. After that, wait for SCW and immediately justify on the Nationalists.
3. Join the war as soon as you can and the Republicans will join your faction (you can use your focus to send some volunteers there to help them survive until then).
4. While fighting the Nationalists, starting preparing a coup in France. Make sure it fires in Nord pas de Calais, as you only need to capture Paris and this province has the closest tiles to the capital.
5. When their civil war starts, the French Commune will also join your faction.
6. Next, kill Portugal attacking from Spain. The tricky part is that their will inevitably call the French, who will easily be able to get higher warscore than you with their superior army...Don't allow that. Fight well, attack over the rivers and take occupations, maybe even try a naval invasion. The thing is that the French love themselves some African colonies and they will take both Angola and Mozabique if given a chance.
7. After that, justify on the British and invade. The combined Spanish and French navy should keep the RN busy enough for whatever the fleet you would be able to scrape together to allow for a landing. Nevertheless, invest in it early on (take your naval focus while you wait for communism to tick up), otherwise even the Portuguese shipyards wouldn't allow you to build enough on time. The British AI will send their whole army to Africa where you will just need a few cheap infantry units to slow them down in the mountains while you run wild in their home.
8. Leave Belgium for last. You wouldn't even need to risk the French involvement like with Portugal, and just naval invade from the UK.
I did the whole thing on historical sometime in 1939 before WW2.
Thanks for the guide on the CK2 achievement. Much appreciated!
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