It depends on your game. Most games have some kind of simplified system to represent terrain, units and battle. Depending on which mechanics you include or exclude, different strategies and tactics can emerge as especially dominant. A simple example is flanking, in some games attacking from the "side" or from behind is ignored, in others it can be decisive.
Studying how other military games, especially those similar to yours, handle this might yield some insight into how to handle this - or in many cases what not to do! For game A.I., not having glaring weaknesses can be more important than trying too hard to make the "best" choices. As an example, in at least one version of Civilisation, if you invaded an opponents territory, they would throw *everything* they had at you to defend themselves... even if you "invaded" by placing several high defensive units on a mountain square (which has a high defensive bonus).
Another important consideration is whether you mind allowing the A.I. to "cheat" in various ways to compensate for the lack of human creativity and ingenuity. This can be a rather simple way to help put the human player at a disadvantage which might result in a more enjoyable, challenging game.
i searched a bit on internet but i couldnt find so much.
What are you searching for? I appear to be getting some interesting stuff just by searching for "strategy game AI".