Hearts of Iron 3 is a grand strategy world war 2 game. The player takes control over a nation-state or empire, controlling the economy, military, and political establishment. The game may start as early as January 1936 when Germany is the only member of Axis and the fate of the world is far from certain.
Hearts of Iron 3 (HoI3) is a very complicated game. Even though the focus is on combat, there are countless non-combat actions the player may take which influence the way battles and wars unfold. The following is a list of the different broad categories in HoI3 which may help to familiarize you with the game.
Combat Modifiers summarize most of the effects of the state of the battle, the unit, and the state on the battle.