On the US Map (not the map of the battle), Press Start to get into the options menu, then go into the Cheat Menu, then enter X, B, A, B, Y. This code makes sure your faction has won the civil war part of the game and unified the continental US. Note: Your faction still has to fight against the Russians in the Battle of Alaska (The Final Battle of the game).

Win Civil War - X, B, A, B, Y