A very detailed strategy game with nearly unlimited gameplay
The game comes with shorter 'Scenarios' - the German invasion of Poland (you play Poland) is my favorite here, and then it also gives you a Campaign mode that they call 'Sandbox' - the sandbox is in fact anything you want to make if it. You can pick where in the world to start (over 180 choices in a 'world like it is today' version, and over 250 choices in a 'world broken up' version). You can set victory conditions that include peaceful victories and time limits, or just go for world domination :)
The official Gamespot review doesn't do the game justice for its depth and detail - yes, it is not for everyone. But if you like detailed strategy - 'real' strategy - then very few games like this ever come along, and this is one of the best.
Now I better get back to my latest game before I lose my upcoming election....