[Arryn]

Originally posted by Sheap:
The AI tends to attack the most weakly defended province. You can't catch enemies by attacking where they are, but once you get the knack you can often catch them by moving to where they are going to be.

I used this exact technique to devastating effect versus Ashikaga in my AAR. I "ambushed" the AI's armies in and near his capital, repeatedly, to weaken the AI until I was ready, and confident, that I could take his capital without fear of an effective counterattack.