The game contains several scenarios covering the beginning, middle, and end of the war and a campaign game covering the whole war. Depending on the scenario, your familiarity with the game, and the amount of control you are taking, a scenario can take up to 50 hours. The full campaign game, even for an experienced player, can take in excess of 200 hours to complete.
Difficulty levels are available, though all they do is make it easier for one side or another.
Every single air group is represented. Most of the important leaders are represented. It is a complete package. The basic goal is to capture enemy bases in order to secure your own position or undermine your opponent's position. Bases allows you to project naval and air power further and further in the Pacific.