For Laurier and others who notice the AI waiting before attacking cities, or not even attacking them. I've played as China and watched Japan's attack strategy: they seem to wait until aerial bombardment has made significant headway -- China has a very large (if underpowered) army and they should not want to take them head on. This can take a very long time because the Japanese airforce is underpowered, and not very much stronger than the Chinese. Also, the strategy is scattered (bomb a factory here, bomb a ship there, bomb an improvement over here, strafe a unit over there) but eventually they tilt things in their favor if their airforce is up to the task. Why they don't use the artillery in their stack is a mystery, but probably for fear of damage. However, if they aren't going to actually attack, it is a waste of money to leave their own borders.
That doesn't explain Poland/Germany/USSR, since Poland is drastically undermanned compared to China/Japan. As for Germany, I noticed in my game as Japan, where China seemingly inexplicably didn't respond to my invasion of their coastal cities and held infantry in reserve in Tibet -- the AI is also aware of other potential threats and does guard accordingly. It may not always do so wisely (guarding Tibet against the UK is a waste since the UK cannot cross the Himalaya except in the east) or adjust strategy if a current war is going poorly, but it seems to have a reason for doing so. Germany may be holding units in reserve to counter UK/France in the west (or Italy et al. in Open Mode).
This may also be a difference in how we humans handle the timeline of Historical Mode -- e.g. presumably AI-Germany does not know that it ideally must dispense with Poland quickly so that the Russians do not grab all of it, and so that it can be ready to attack Norway a few turns later. But as I said above for my games in Open Mode, the AI is likely guarding against other potential threats.
I do wonder if the AI overvalues forts -- sure it is better than defending open ground, but unless it is a chokepoint it may not be wise. If forts had minor AA capability it might make it worthwhile to hole up there, though there would still come a time strategically when they would be better off defending their cities instead.
To follow up on the potential defense glitch mentioned above, I also wonder now if it extends to air units and naval units. I got 'curious' results, losing stronger/promoted Japanese Zeroes to China's early-level air units. Similarly, battleships being sunk by cruisers. Sure, some randomness is to be expected, but it makes me wonder.