I was trying to take Egypt down a peg because they are obsessed with sowing discontent in my empire. From a military point of view I had no problem with it--I took 6 provinces and won every battle. However, I couldn't get any settlement out of them because my warscore was too low--as far as I can tell the problem is he would keep sending a small force to siege one of my provinces. I would have to pull an army off siege duty to take care of it. I'm not sure if it was the sieges or the abandoning of my own sieges but I only had an 8% warscore at that point.
The AI seems to have a basically perfect ability to avoid unwanted combat while moving, short of blocking every path (for which I didn't have the forces) I see no way to stop what he was doing.