Boy the Ai is Duuuumb.

[Q]

That is not an AI problem. You may have the same situation if you are the invader in strategic combat. The placement of the planets/moons and the inability of the hard coded SE IV movement program to calculate a ship route around an obstacle is the real cause, and sometimes quite annoying. But if the AI (or you) attack again the placement of the planets will be different and the invasion might succeed.