Both campaigns feature Primary Objectives, which are the basis to completing the mission. Failing a primary objective results in a game over screen and the opportunity to restart from the last checkpoint. Checkpoints are often found in between primary objectives, much like a "Mission Update" or something similar. Completing only primary objectives yields a bronze medal.
Secondary Objectives are listed objectives that can yield better results, or unlockable units. These unlockable units can replace existing ones for the mission – for example, a player can choose to have all B-17s turn into B-29s for the mission, which are tougher and faster bombers.
The Hidden Objectives yield even better results, and completing these (plus the Secondary Objectives) makes for the best possible mission outcome. This is the only way during the campaign to unlock the Iowa-class battleship, for example.
The American campaign begins where Midway left off. Starting in the Solomon Islands and working its way to Okinawa, the US campaign follows a few unnamed characters exchanging dialogue. The Japanese campaign is identical to the American one in nearly every respect. Both campaigns have a pair of cutscenes which give a minuscule storyline – a soldier going off to war and, at its conclusion, returning to his significant other.