This is hands down my all-time favorite strategy game, thanks in no small part to me being a big World War II buff and a fan of strategy games. I've literally have spent hundreds of hours playing it, and I still find it fun today.
You can begin your campaign from either 1936 or 1939. You'll have nearly 200 countries to choose from, including the major powers at the time: Nazi Germany, the United States, the United Kingdom, the Soviet Union, the Empire of Japan, and the Third French Republic.
The potential for creating your own alternate history within this titanic conflict is practically infinite. Want to conquer the UK before invading Russia as Nazi Germany and ultimately win the war? Go right ahead. Turn America into a fascist dictatorship and join the Axis? Not a problem. Start World War III as Russia and go to war against the Allies once the Axis are out of the picture? Nothings stopping you.
You can potentially command a military sizing in the millions with support from tanks, aircraft, artillery, naval vessels, and, with sufficient research, even nuclear weapons. To keep your war industry up and running, you'll construct factories, refineries, roads, railroads, and dockyards. You can organize your divisions into armies or entire army groups under the command of your hand-picked generals and field marshals. You'll even be able to design your own divisions from the ground up and decide what weapons or equipment they'll be issued.
You can research new weapons, vehicles, tactics, industrial and research methods, and others to improve your performance against your enemies. You can also implement different doctrines for each military branch that differ in the bonuses they provide. You'll also be able to choose progressive options called focus trees, many of which are based on historical events, both real and alternate. They all vary widely in whatever perks they grant you and how they may reshape history and the course of your campaign as your nation grows, so choose them wisely for the best benefit.
You can spend political points on vital decisions, such as economic policies, war plans, justifying war goals, fighting rival political parties and others. You'll also spend them to appoint government officials, joint chiefs of staff, employ production companies, etc. You must spend them carefully to achieve maximum efficiency in your strategic goals.
Hearts of Iron IV is a masterpiece game, and the fact there are still updates and DLCs being released for it speaks volumes of its success. Plus, the bugs and glitches are at a minimum, so you can expect clean, unimpeded gameplay. There is a lot to learn, and it takes time to get used to the game's features, but it's worth it. Try it today if you're into strategy games and World War II.