I've never actually seen the AI use chemical weapons on anyone. I've seen them build the units, but I've never seen clear evidence of actually depopulating a city. They've definitely never done it to me, despite the loads of chemical attacks I've made on Aliens that I want to eradicate. It's possible that my offense is just too good, that they don't even get the chance. But they don't even use chemical weapons on my units destroying their home territory. I'd expect a chemical counterattack sometime, and I've never seen one.
I've rarely seen the AI build a chemical weapon. In fact, I've wondered if they've only ever done it, because they captured one of my X units and got a prototype. Making Nerve Gas Pods a Tier 1 tech hasn't changed the AI behavior. They are not adding a cost to their builds for a "useless" capability. And if a real human player wants to add this cost to their builds, that's their prerogative.
If you are playing a human faction, and you use chemical weapons with impunity on another human faction at the beginning of the game, you are going to die! Planet is going to take a horrible vengeance upon you that you will not live though. Or if you do, it'll cripple you so badly that other factions will nuke you. Here's an
After Action Report of exactly this sort of thing happening. It ain't pretty. I quit that game in disgust. I've never survived a chemical weapons triggered mindworm apocalypse, and I've tried about 5 times now. It is one of the dullest things you can possibly do in the game. Imagine every turn, you fight mindworms... and every turn they are replaced.
I am not sure if Repealing the UN Charter cancels the global flooding and mindworm vengeance. That's worth testing and maybe even doing an AAR about. You do
not get mindworm apocalypse when you're a human attacking an alien. This is presumably because the UN Atrocity Prohibitions do not apply to Aliens, and
unpresumably because Planet bleeds its liberal heart for humans only. A surprising level of awareness of human warfare conventions from Planet, for a sentience that hasn't awakened yet. You'd think the same chemicals spilling on the ground would get the same results.
Another serious consequence of chemical weapons for a human faction, is Sanctions. In that AAR I had 1000 years of sanctions, longer than I'd be playing the game! It's not easy to get the votes together to Repeal the UN Charter either. If you want to do it early, you'd need to do a lot of bribing. That costs a lot of money or tech. It might be a wash compared to just taking the Sanctions. I'm not sure if Planet's attacks are based on Sanctions or not. The humans and the AI don't participate in Commerce together, so that
could be why Planet's behavior is different.
As an Alien, I haven't typically used chemical weapons on humans. For one thing, human cities depopulate to 1 when you conquer them anyways. Granted, you don't have to conquer a city to depopulate it with chemical weapons, you just have to win a battle. An air campaign liquidating cities is the usual drill. As a human I've done it to the Aliens plenty of times, but not typically the other way around. The main reason to use chemical weapons is for the offensive advantage, and I suppose I haven't needed it.
As an Alien, I haven't typically used chemical weapons on the other Alien faction. Alien cities are the only ones I can get a substantial population from, so I do not necessarily want to depopulate them. Aliens also don't do Commerce, they have an energy grid which makes more money the more cities you have. So taking over all possible Alien cities may be in your best interest.
In short, my attitude about chemical weapons is: you want some of
this? Knock yourself out!
