This game lets you play as any country during World War 2. I played the base game plus all expansions (SF, FtM, TFH).
You can manage every aspect of the war, from diplomacy to espionage to research to actual combat. You can even try to stay neutral and sit out the fighting if you want, but what fun would that be?
There is an enormous amount of complexity - casual gamers beware! If one part of the war is too tedious for you, it can be set to be automated. For example, rather than personally overseeing every trade deal you can check a box and let the computer handle all of your trades. The computer is not as smart as a person and it can't metagame but in most tasks it does an adequate job.
Personally, I'm a WW2 history buff and I love to wrap my head around complex rule systems, so this was right up my alley. This is one of the few games I've played where I wrote out multiple pages of notes to plan my strategy.
For my first game I decided to play as Canada, which is a good beginner choice because you're safe from imminent invasion and have a decent (not overwhelming) amount of resources to play with. Just like history I joined the Allies. I sent troops to help France, getting my first taste of combat. I lost of course, but learned a lot and even got to stage my own hectic Dunkirk evacuation to get my battered troops out of harm's way. Later my rebuilt forces kicked the Italians out of Africa and captured Sicily. An amphibious invasion of mainland Italy failed, but when my British allies successfully invaded Nazi-occupied Greece I joined them and had great fun. By 1944 the Soviets had overrun most of northern Europe, with the western allies in control of the Balkans and Africa. I quit at that point, since finishing the war against Japan was a naval battle and most of my fleet had been sunk.
Next I decided to play a totally ahistorical game. I chose Brazil and decided to ignore WW2 and try to conquer South America. I built a huge army of militia and other low-quality bullet fodder, successfully overwhelming my neighbors through sheer numbers. Alas, my flagrant violations of the Monroe Doctrine had attracted the wrath of the powerful USA. Shortly after I captured the Panama canal the Americans arrived in force and their tanks proceeded to pulverize my hapless militia. Still, it was fun, and I learned about the importance of anti-tank weapons and combined arms.
My next game came about while fooling around with the espionage tools. As Germany I helped the national socialists win the 1937 election in the Netherlands. It was a huge electoral upset, sweeping away the liberal democracy and replacing it with a totalitarian state. Accusations of fraud and vote rigging were completely unfounded (tee-hee!). I thought, wow, this is so cool, I just changed a government! But the only thing cooler would be to actually play as this new government. So I saved the game and rejoined it as the new extremist Netherlands. I joined the Axis, but refused to join Germany in declaring war on the UK or Soviet Union. Instead I conquered a bunch of neutral nations, and acted essentially as a resource "banker" for the Axis faction, as the Axis powers are resource-starved in the mid to late game. What fun!
I've played many other games but the stories are too numerous to repeat. I'll just say if you like military strategy you're in for a treat!
I have a few criticisms. Despite the manual being 80 pages long, it wasn't enough. There are many things in the game which aren't well documented. HOI3 is even more complex than Civilization V, but it lacks the helpful "Civopedia" feature that game had to quickly look up how something works.
For example, in one game I was playing as Finland and planned several years in advance to declare war on Germany and then launch an amphibious invasion of Norway. I spent considerable time (years in-game, hours in real life) building up my fleet, but when it came time to attack I found I couldn't move my ships to Norway. After much puzzling I discovered that since the Germans owned a specific province that the Danish straights were blocked to me. There was nothing in the manual about straights being blocked, and nothing on the map to illustrate that they were. So all my preparations were for naught. Lesson learned: Save often! Or better yet, set the autosave for every 6 months so if any similar nasty surprises happen you'll be prepared and maybe able to recover.
My other criticism is that the supply system is dumb. For some reason ports and airfields don't stockpile supplies. So if you re-base your planes or ships to a new base you immediately run out of fuel. Even worse, all supplies are distributed from your capitol. So if you're playing as the USA don't be surprised when your troops in California start starving to death because their supplies are inexplicably drawn all the way from Washington DC (and each province the supplies go through results in a supply tax).
I would think the solution to this problem would be to create supply dumps at appropriate locations. Unfortunately Paradox's solution to the problem was to create an "Arcade mode," an optional mode in which all supplies magically appear wherever they need to be all the time. But this just creates new problems because a big part of the military strategy is about cutting off enemy forces from their supplies, using bombers to shred their logistics, ect. With arcade mode you lose all of that, so I ended up sticking with the default supply system despite its flaws.
Overall I recommend this game to WW2 history buffs and military strategists. Casual gamers should stay away though.