Originally Posted by Kobal2fr
"If they have more than 3 times as many men as us on their frontlines, and their total strength is more than twice ours, and their economy is much better, and they're our strongest neighbour, then raid and want_allies_against_him".
Now, don't take this at face value, because obviously it's a low priority decision, and the AI will deal with more pressing matters first... but nevertheless, it's geared to annoy the strongest around if possible and there's nothing else to do/no other war going on.