HOI4 it’s a game about Word War II. There aren’t 100% historic accurate but developers but the developers have done their best to make the game believable and accurate there are many events built into the game that allows you to do exactly what was happening during World War II, but if you want you can take an alternate route…
… if you want to repeat history, you can. If you want to try what if, you can. And that is why I love HOI4 and why is HOI4 so popular. Another thing that affects realism is the AI setup, which we’ll talk about below.
HOI4 AI settings
In HOI4 are two AI settings that you could set before the game start. HOI4 has a very simple setup either you want believable AI or you don’t. And if you decide you want alternate AI then you can choose the details (I’ll write about that below).
And it’s these settings that make HOI4 a game that will keep you entertained for hours and hours.

If you want to play an alternate history, you can even choose which way a country will develop – the AI then tries to pick a given line from the decision tree. The selection is named AI behavior.

But, it doesn’t necessarily mean that if you choose “historical AI” WWII will unfold the same way. Some events may play out slightly differently though, for example when I play in Czechoslovakia Germany always takes the Sudetenland +/- at the same time. Similar cases are reported on the official forums, but even so, the game is very accurate.
Other custom game settings
There aren’t just AI settings that could affect your game there are sliders about “major nation buffs”. In these sliders, you could add more strength to major countries which are in the game. In the next picture you could see buffs which you could gain:

There are buffs like more political power gain, or you could get more strength divisions because you could have a better division recovery rate or faster research speed. Remember, if you’re going to give a buff to yourself so give it to some of your opponents so you don’t have too easy a game.
Second, think what you could set up is “game setup”. In these options, you can set what state each country will be in. One of them is for example whether Britain will have as many colonies as it had before the Second World War or whether the Soviet Union will be united or not.

The great advantage is that under each option Paradox has prepared a description of exactly what you choose and you can customize the game in detail. We are talking more about alternate history.