So, I started up a game as the Duke of Apulia, and things went pretty well taking out my neighbors and forming the Kingdom of Sicily. I had secured multiple-sibling/daughter marriages with the kings of England, France, Hungary and even Scotland, which made my ability to expand quite a bit easier.
Then 1086 rolls around and out of nowhere the Fatimid Caliph decides to call a Jihad on me. I call up all my allies to help defend, and all but one say they are on their way. At this point I can muster around 3500 troops total, as 5 provinces still suffer from the "recently conquered different culture/religion group" penalty. Ok, rough time but I can work with it, I think. Poor deluded me. A few months later the ships arrive, along with the armies of the Caliph. A pair of 12-13k doomstacks, plus a handful of 2-5k stacks. By this point Tuscany has sent me 1500 men, and I can see the first English ships arriving, giving me a total of just under 8k troops, who promptly get decimated by the smaller of the two enemy doomstacks (yes I used my tactics right, both leaders on the flanks had Flanker commander bonuses, and the center had Inspiring Leader - I only killed about 1k troops total). Due to mass assaulting, within a year I had lost the entire peninsula and suffered a game over.
My question to those who have been targetted by Jihads/Crusades before is: what can be done to help ensure survival of them, aside from rolling over and giving up? I know they can fail, as I regularly see Jihads against Anatolia/Armenia called off, even during a Byzantine civil war. Is there some basic logic to the choice of targets the AI uses, like say had I not taken Malta and kept my borders farther away, would it have picked someone else as a priority?