These are the routines that enable the computer to operate as a worthy opponent for a human player. These AI routines are not as complicated as they might appear. AI routines need not take up a lot of space in the program. Doctrine is what drives AI, doctrine is the standard procedures that armed forces use when in combat.
Every armed force has a doctrine.
Some doctrines are more efficient than others, but that's something you have to dig up in your research. What the doctrine says is not always what the troops do. You have to do more research to find out how the troops deviated from their doctrine, to what extent and how often. There is often imperfect information. That is, the commander is often not sure of the status of his own troops and is even less well informed about the enemy.