So I started WW2 over Romania.
Hearts of Iron 4 is a grand strategy game about the greatest generation back when they actually did things other than complain about "Back in my day!" nonsense. In true Paradox fashion the game allows for some pretty silly things, such as taking over the known world as a Communist Canada, but it has a bit of an issue in not letting people make their own way with the major nations, Germany can never be a naval superpower for instance, no matter how much you want to be one the game locks your naval doctrine to one of commerce raiding.
In motion HoI4 is pretty similar to most grand strategy games, which is to say you need a PhD and a cheat-sheet to play it, but if you can navigate the various menus and tech trees you will be rewarded with an industrial interface that I suspect should have a spreadsheet. Diplomatically things play out slowly and methodically, you have to take a year of your time to fabricate an excuse to go and invade some neighboring nations, while they have time to go beg for alliances and build bunkers, so you best be ready for that big push across the border. Once in conflict the game is very accurate the actual needs of an army conducting an invasion, you'll need to carefully manage your industrial capacity and manpower to keep armies stocked with goods, weapons, and soldiers as they press onwards.
Now managing a mass scale invasion of Bolshevik soil is quite the undertaking, you'd almost be a fool to try and do it all on your own, in real time, which is why the game has a handy army grouping function that allows you to be Hitler the right way and just let your generals handle the whole invasion business, army groups can be set to defensive, neutral, and offensive as they push forward, and the general AI will automatically shift units along the designated frontline you assign him to as needed, hopefully minimizing the rampant losses of expensive tanks. Planes on the other hand are actually fairly simple, you assign a region to your squadrons, give them missions, and they will automatically carry them out for as long as they have planes. It's actually a very strange and fascinating ritual watching battle lines shift as the AI launches offensives both successful and unsuccessful, at least if you ignore the fact that each loss costs you a dozen or so tanks that take a month to replace a thousand or so able bodied men.
The game does make an okayish effort to let you rewrite history though, with Germany being able to cement an alliance with the USSR instead of invading, complete development of the atom bomb before America, it even allows you to design your own tanks by mixing different attributes as you see fit, after all there's nothing dodgy about a light tank with a 12.8cm gun. and even making major changes to the governments of key players in WW2, such as the aforementioned communist Canada.
Hearts of Iron IV is essentially a Paradox grand strategy game that trades incest and murder for tanks and rockets, which lacks the charm but generally makes for more engaging combat.