The AI for two person games can be created using the alpha-beta (AB) algorithm. AB generates ALL possible moves in the future until the game has ended but usually that is impossible so you have to use an evaluation function which looks at the current game position and tries to calculate which player is winning and by how much. Obviously creating a good evaluation function can be very complicated. The "heart" of the AB algorithm is the evaluation function.
(Sorry for a little bit of review but I just wanted to make sure that everyone is on the same page.)
While AB has been very successful when applied to chess, AB is not without its weaknesses. AB does not have a "game plan", such as trying to maximize the value of Wrath of God. AB just looks ahead and picks a good move. The AB algorithm may appear to have a game plan which would give AB some "human characteristics". The best kind of AI is both challenging and realistically acts like a human opponent. (Depending on the specific game the AI may do something that a human player would never do which ruins the illusion of playing against another person. And yes I've had this happen to me while I was playing Forge.)
While AB does not do any planning, there are other algorithms that use goal-directed reasoning or planning. Unfortunately these types of algorithms seem 100 times more complicated than AB since they need more information. Reasoning and planning algorithms need to have knowledge about the game and the ability to generate or choose from a variety of plans.
Through trial and error, AB can be combined with planning algorithms which would allow the best of both worlds. Maybe AB could handle combat while the planning algorithm could choose an overall plan such as playing a creature every turn or effectively using Royal Assassin.
If Forge 2 ever got AB working, it would be fun to let people tweak the evaluation function to see how it helps or hurts the AI's performance.