I see, vs AI its a bit tricky I always feel that it sort of cheats and picks the best province to attack(the one that I didn't decide to move into)
. I'd suggest going to C hoping that hes gonna attack there.(Thats is if you are in war with the AI but since it attacked province of yours you are
) This way you do 2 things prevent him from taking any provinces of yours(A has fort so he cant take it in one turn) and if it goes for the fort he is going to remain trying to take it so you can destroy his army and then move on to take his territories.
The other option is to attack his province but I find it as a fifty-fifty chance, but every time I do they seem to get lucky and move their army first taking my province C and moving futher in my territories raiding me(and annoying me), so that option feels a bit useless.
Hence I recommend option 1.(then if he doesnt attack C or A, you can attack him from C.This gives a higher chance of intercepting his army, if on the next turn he decides to attack C, If he decides to attack your fort you'd get B and he will be stuck on top of your fort so you can kill his army)If you manage to destroy his army first then you can move on to conquering him without loosing provinces and being raided by 10 small AI armies.