The AI might be a little too willing to nuke worlds.

[zenopath]

This is kind of an interesting thought for me, because it isn't immediately obvious why this is a problem.

First consider it from the point of view of the player, a player who uses bombardment to nuke worlds is at a serious disadvantage because recolonizing every world would be expensive and you lose the bonuses of capturing population. Moreso, if you bomb once, you gain what appears to be a permanent negative penalty from that race, which makes them harder to keep happy, which also means you require more garrisons which makes it harder to build up enough troops to avoid using bombs. In short, it makes sense from the POV of the player never to use bombing and just brute force capturing planets with sheer numbers of troops. It might be a little expensive to build up a mass of ground units, but once you do, you usually don't lose many capturing new worlds, so there's only a big initial investment. Using bombing not only hurts your future productivity, but it also makes holding on to captured worlds expensive and might require recolonization if you decide to go for scorched earth rather than ¨soften em up then invade¨ bombing tactics.

For the AI it's kinda a different story. They usually have a LOT of colony ships, are often fighting a losing war, and it benefits them to scorch your worlds rather than try to conquer because you might come back and reconquer them. The AI does sometimes use troops, but only on lightly populated worlds. On populated worlds, it will nuke everything to the ground rather than try to invade, which can be pretty crippling to you as the player. It will also send small raiding fleets to hit vulnerable planets, which is a tactic I can't use as I can't commit a troop force to a throwaway attack, but rather can only concentrate them in one or two big fleets.

But, here's the thing, The AI doesn't ONLY fight with you. It also fights other AI. When that happens, the AI's tactics often result in both sides losing massive amounts of population and worlds. If both sides are scorching each other, by the time they are done and agree to end the war, they've both taken massive damage. If the AI were more hesitant to bomb worlds, they'd do less damage to each other. This means that, while there's some benefit to the AI to use scorched earth bombing tactics on you the player because it hurts you more than pure conquest... it definitely hurts the AI in any war with another AI because both of them are scorching each other.

It's almost like a prisoners dilemma, as a player, I don't bomb, so I risk taking more damage if I let the AI near my worlds, while in the short term, the need to use troops makes my victory against the AI harder, even if, in the long term, I benefit more. The AI's short sighted use of bombs is really bad when two AI go to war though, as it basically ensures that the AI never benefits much from fighting another AI in the long term, since both sides will likely lose a lot of population, which in the long term greatly weakens the AI's late-game power overall.

[sven]

The AI's short sighted use of bombs is really bad when two AI go to war though, as it basically ensures that the AI never benefits much from fighting another AI in the long term, since both sides will likely lose a lot of population, which in the long term greatly weakens the AI's late-game power overall.

I think this analysis is fairly sound. And the situation is going to become even worse for the AI when the new fallout mechanics Arioch and I are currently testing for DLC2 go in -- as that makes aggressive bombing even less wise in many situations.

Part of the reason I made the AI such aggressive bombers is that there were low-level problems with the fleet routing code that made it difficult for AIs to reliably mass enough transports to take a planet. However, about a year ago, I wrote some patches to the fleet routing code that may have helped with that issue. I could now try making a variant AI strategy that's less aggressive about bombing, and builds larger fleets of transports. I suspect that's worth experimenting with.

It's less simple than it might be, because as you say, building the large transport fleet and minimizing bombardment damage is only really wise if you're fighting a war that you expect to win. And the AI isn't that great at predicting its own odds of success. But, I think there are some simple heuristics I could write here that would be worth trying out.

[orvarth]

adding non-agression pact and the possibility for ai ally to cancel alliance , diplomacy penalty for bombing could lead in losing these pact with other faction .

[zolobolo]

Bombing extensively is only an issue if not done till total destruction or till only 1 pop remains from player side:
1. If colony is entirely destroyed, there is no issue with moral at all (nor minor species) nor cna the AI take back the planet and is thus denied range which is a huge advantage when you player if fighting a stronger force
2. If oly 1 pop is left, it can be "pacified" by a single tank battalion - moving 1-2 core pops onto the new planet ensures they will never outpace their growth and thus result in rebellion even if the single tank is later tken away

The above two reasons leads in my case to always bomb excessively - if not done so from he beginning and all the time, other races will be upset when you do need to do it and will rebell - something they cannot do if they dont exist or are too few in number due to #2

Fallout will definitively help to put a break on the above so the players do not have such an easy way of dealing with huge protected planets and other pops integrated

Now in case of AI it is true that they are weakening each other but they also deny access to each other on top of them, leading to low amount of territory being captured.
Fallout will make things even worse here by introducing long-term negative effect to AI planets bombed so there for sure needs to also be less inclination from the AI to bomb going forward

[zenopath]

I think there are two basic options. You could add in a “selective bombing” type to the invasion screen, costs 2 or 3x as many bombs to use, but it has a much lower civilian population casualty rate. That way, if the AI has massive fleets it can use this against the player or another AI to capture worlds using only a few transports and only kill a fraction of the population normally required. Basically, it would offer a way to kill militias without killing a million pops.

The second option is a sort of “civilized war” where the AI starts out choosing not to bomb enemy targets until it gets bombed. Basically it would be a "If enemy has used 0 bombs on me, I will not use bombs." This behavior would be the default between AI's when they don't hate each other that much, and how they will fight the player if you've played a game without bombing them once. You could code some races to adopt this behavior, like the phidi, and maybe have the more evil races, like the tinkers and gremarks generally default to "non-civilized war." This would at least make it possible for some AI on AI wars to be less destructive, while also punishing the player who bombs an AI by having that AI switch to a "full bombardment" stance. The toggle could also be set so that at a certain level of negative reputation aliens will use bombs on each other, but at a more neutral reputation, they won't unless the other side bombs them first. Since generally speaking the AI rarely hate each other as much as they end up hating the player, it would reduce AI on AI damage.