AoD is a mod for Hearts of Iron 2: Doomsday. And while it's 9 years old, they're still working on it, which is rather amazing.

In AoD you pick any of the provided starting dates (earliest is 1-1-1936), pick any country and you run it - sort of. You can engage in diplomacy, politics, research, espionage, trade, production and warfare. At certain historical dates events will fire for some countries which generally offer a historical and ahistorical option. For instance, as the USSR you can decline to carry out Stalin's purge of the army. This will benefit your research and military strength, but at the cost of a gigantic dissent penalty (50%) which can cripple your industry.

It's basically a "what-if" simulator. What if th left won in Spain and, say, joined the Allies? What if Barbarossa completely succeeded or failed? Or maybe what if Brazil or Turkey got involved? There's a substantial amount of skill and knowledge involved, but you can do some fun things. And of course you already know what's going to happen. As the USSR you can build a huge army and powerful fortifications and stop Hitler much earlier.

AoD does have a big problem: the interface. There's no proper overview of all your provinces with all information you might want (like resource production). Budgetting involves messing with terrible, clumsy, poorly responding sliders. Very little information is given about many things. And construction can't be reduced or paused from the province screen.

Also, smaller countries are overly punished by the research system that relies on Industrial Capacity. And it takes so much time. A single game can take dozens of hours. But yeah, the interface is what really killed the game for me. So i'd recommend it, but only if you can live with quite a bit of tedium.