Still on my old computer but just had to share this screenshot from my current game as Germany (small map, continents, standard speed, prince level). I took advantage of a Heroic Age to attack Japan to my North. I've taken Osaka and Tokyo. I got a military starting an attack on Takamatsu and my army that took Tokyo is making its way up towards Kyoto although I think I might divert it to the Japanese coastal city on the left of the screen (under Mobilization). But check out my allied city-state Vilnius sending a pretty good military to attack Japan's capital, Kyoto.!!! Looks to me like the AI can do pretty well when it wants to.