Oops?

AI can surrender.

I scared them to death? That is very... unsatisfying...

Yet fairly realistic - why die needless and practically preordained deaths when they can hopefully live on as your vassels?  It's uncommon for a surrender to happen that fast, but as noted it does happen.

Well I basically had the fleet cap... in a single fleet. Plus all the capitals. Did I miss a message somewhere though about them surrendering? It was just so unexpected...

So assuming I'm playing with players or an AI that doesn't surrender, what are the victory conditions for the game?

Pretty much kill planets and colonizing ships.

Are the harder AIs more difficult to scare?

ironically no

though you can modify the AI to not surrender as much, it is in the gameconstants file way down at the bottom, you will find the gameconstants file in the gameinfo folder of your sins game, if playing entrenchment then it is sins/entrenchmemt/gameinfo

The AI is very prone to surrender when it lacks the funds to rebuild its fleet to a comparable level to your's.  It has nothing to do with difficulty, and everything to do with the AI simply being out of options.  In all honesty, every time I've seen the AI surrender they've been horribly outgunned and outnumbered.  Sometimes they have enough units and planets to make a good last stand, but as satisfying as it may be for you to crush their last strongholds it's really already over anyways.

As I've said before, the AI needs to get progressively more "desperate" and aggressive as the player builds an advantage against it.  Right now the problem is that they spontaneously surrender due to an immediate disadvantage, but the causes of that disadvantage didn't appear spontaneously.  We need an AI that recognizes that it's losing the economic, colonization, or tech war and starts being more aggressive.  On the other hand, I often find on smaller maps the AI will never surrender and I have to chase it down to its last dead asteroid to win.  It's silly when it has no units, no frigates, no structures, just a planet with 6000 hp and it forces me to bombard it to death with my one capital ship.

I usually avoid this problem instead of multiple stars, I use the map where everyone is in the same galaxy

You can continue to play after your victory and go in and mop them up. Just be aware that they wont be building anything new, and they won't leave their grav wells at all, so basically it's like kicking someone who's already on the ground and has ceased fighting.

Kind of pathetic, really.

But yes, I agree - having the game suddenly go POOF! and give you a victory screen when you thought there was something left to do can be a bit disappointing. My suggestion is crank the difficulty to Hard and take on multiple enemies. Just so you know, Unfair AI is the same as Hard AI except that it cheats (more resources or some such).

I looked in the file you mentioned but was unable to locate any line that appears to effect the AI's surrender chances. The closest thing I could find was a retreat threshold. Could you tell me what line to modify exactly?

Pretty sure there's none for surrender, just the one for retreat.