[Ygorl]

At least in Dom2, the AI would tend to pick whom and when to attack by comparing the number of troops to the number of provinces for each nation. If a nation had lots of provinces but few troops (regardless of their power) it would be a ripe target for the AI. You could fool it by hiring lots of PD (those were counted) or lots of garbage troops, thereby convincing it that you weren't worth the trouble of attacking.