[Gregstrom]

Actually, I think it could be more complicated than that.

The manual implies the spell goes through commanders in the province in order, and checks against each of them in sequence. If so, a horror marked enemy commander who is 10th in the list only has a 1/2^9 (1/512) chance of being attacked. If the 9 commanders above him in the list are friendly, they would have the best chance of being attacked.