nonhumankind wrote:

Playing on nation difficulty with ai bonus removed, I noticed they are no more aggressive than metropolis, and would not even try to steal away my unprotected cities (with no wall and no population!). I advanced into Spain at turn 107 at endless speed by just two armies of 4 warriors each, but their cities are well built with 3-4+ districts, so clearly they could produce units and easily beat me, but choose not to.

Is there a mod to fix this? or need custom AI personality?

The "aggressivity" of the AI depends on several factors, but which persona was the opponent? There are some AI that tend to be more pacifistic than others.
But for the not building districts that is something strange, would you mind to share some save + diagnostic files?
Save >> \Users\[username]\Documents\Humankind\Save Files 

Diagnostic >> \Users\[username]\Documents\Humankind\Temporary Files 

no I meant they built a lot of districts. Their cities are much more productive than mine, therefore they could have been far more aggressive.

I chose random persona this time and all of them were not aggressive. Even the Achaemenid with 3 vassals had just 2 cities.

How is your military? If you are very strong, they will run away. You basically won't see them.

But if you don't have one... they certainly do make all sorts of transgressions. Ransacking, marching through your territory like it's nobody's business. Especially when they are expansionist - Early Modern era has 3 expansionist cultures (Spanish, Inca, Ottoman - very historical too!). There were armies everywhere.

I agree with Daarkarrow, too, you can change the personas and they do act very differently.

All my population are in units. I have no defense since population in most cities is 0.

Because a few games later I figured out this is the best way to win. If I don't use mods to add city cap, I just keep liberating their cities until they have nothing left.

There are aggressive persona I put, but they're nowhere as aggressive as me.

I've never found AI passive, but I play with strongest AI Personas from the Amplifiers and on a humankind difficulty. Also, the scenario you described seems like a no win situation for your empire, because winner in this game is determined by fame, not by the elimination count. With your approach I can only see you getting military stars past bronze.