They are not mad?
GalCiv2 Forums
So, today I have the Peacekeeper mega-event happen to me.
At first I was thinking that I am doomed.
In fact, they blew a PreCursor Ranger into smithereens! (that belonged to the enemy).
But even after that, they never went after me at all...
100 turns passed and they have not touched me at all.
What is going in and how to get them angry?
Trying shooting at them - that'll do the trick.
Actually, it's a known "bug" (possibly a deliberate feature, though) that the Peacekeepers don't always come in at war with everybody. The Galactic Civilizations wiki mentions in their article on the Peacekeepers, and I've seen it other places, though it didn't happen on the one occasion I met them.
Like I said, if you want them to become hostile, just attack them as you would any other race.
I believe they're only supposed to attack things with weapons on them, but it's been so long that I don't remember if they actually avoid combat with you if you avoid combat with them.
I see.
Well, despite the fact I an not a war with them, I still decided to attack them for kicks and giggles (and because I'm bored).
But I'm am winning my battles against them thanks to Fleet Warp Bubbles, Starbases, and the fact that the peacekeepers did not bother with defenses other than the shields technology (which cause their ships to have nice holes thanks to my Gravatron Drivers!)
Consider yourself lucky-in one particular (DA) game I ran into 1400 attack/700 defense Peacekeeper ships (yes, that's way beyond what we mere mortals can do)-and they were roaming around in fleets of 15-20 ships a piece (also beyond the human player's limit-or the AI's, for that matter).
I believed this megaevent was only in TA because I didn't have it (over 200 DA games) since I played GalCivII...until now. The message said they would only attack ships which were outside their own territory but they are attacking my ships on my planets in my own space.
The Peacekeeper fleets have 35 movement points and 2500 shield (I didn't bother remembering any other details 
); so much for my stealing the Doom Ray tech on one of my invasions. I'm only just surviving a game with free-for-all wars (more than 2 races all at war with each other), so I will continue trying to win through diplomacy and alliances.
EDIT: actually that was one of their weaker fleets...
EDIT#2: ...and that screenshot fleet is also 75% weaker than their shield 15000.
Each fleet seems to have chosen one or two star systems and sticking there, only destroying anything which goes in or out.
Good Lord! Is that true?
Then they are the most powerful ships I EVER seen. 
Unfortunately, it matches my own experience - since the Peacekeepers will always show up with technology that's ahead of the game leader, if you get them late enough in the game they get tech that makes the Dread Lords look like cavemen. I was already going for a tech victory the time I saw this happen, so it didn't hurt much, but there would have be no way to ever fight the PKs.
I got them again today! (DA metaverse).
This time (from what is visible to me) they are not attacking me anywhere but they are attacking everything else. Consistent with the megaevent message, they are attacking ships outside their own combat zone (about 50% of the other races' planets are within my own territory; those are being attacked too).
(EDIT 28Mar2012:) later in the same game... 
I guess they don't have an economy so I didn't try to take their economic treaties. I didn't need to get them to attack anyone, but it seems they are "at peace" with me. Eventually they settled for giving me 256 BC! Surely the worst possible begging diplomat?
I think they will be back because in my current game the Drath also keep begging help which I now repeatedly reject.
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