Getting two AI's with Alliance to break their treaty.

Why not just ally with the other one and win that way?

There is a chance that if you go to war with the second AI then the first AI, your ally, will side with you against the second. I'm not sure what determines which way they'll go. I'd recomend making sure your military is stronger and you're doing more trade with the first AI than the second AI is, just in case one of those is the determining factor.

The only other time I've seen an AI alliance break up is when they got the random assassination event.

~SDC~

Peter Weisberg,
I gave your suggestion a go. But my trade with my ally (all 10 trade routes plus module assistance) did not have any influence on there decision to defend there ally. I was also on par with there military if not a little stronger. I was also conducting a massive amount of destabilization and espionage. None of this made a difference. Im starting to think that the AI's will always have preference towards there long standing ally regardless of total percentage of income they get from you. Alignement for both of us was good. Any more ideas?

If possible, hit one of the allies VERY hard in a single turn. Wipe them out with an "alpha strike" if possible, or just weaken them badly. Then sue for peace that same turn. Relations with the one you attacked will be cool to hostile, but the other one doesn't seem to notice.

If you take a few of their planets with the most influence, and you have the majority of the influence starbases heavily mined, you will start to see the nearby planets you didn't conquer militarily start to flip over to you without a fight.

im not 100% sure on this, but im pretty sure when two races go to war and a race is allied with both, it sides with the race being attacked.

squirrelpire, I think you're right, but this can produce weird things: I A,B,C,D are allied and I am allied with B,C,D:
I attack A. As an answer, B attacks me. Now, C can attack B who attacked me or me who attacked A. Here, C's decision looks like random.

One time I was allied with the Altarians, but was far ahead of them in military technology (Overlords vs. Dreadnaughts) and had 3 fully mined military resource bases, so I wanted to take them over for a military victory. They were allied with the Torians, who had a single planet over in the corner. I still wanted to be able to trade with the Torians, so I sent all 10 trade routs their way, got an alliance, and attacked the Altarians soon after. Torians sided with me. So they don't always side with the attacked.

~SDC~

If that's true, you could provoke a war by destabilization or parking your ships in the enemy's territory. They declare war on you, and your ally declares war on them.

Here is something interesting to check:
Yesterday's game I played.
There were left Torians and Altarians plus me in the galaxy.
Okay. Situation was like this:
Torians and Altarians were allied, for a long time. Actually I think their alliance was quite perfect (only assume so).
Myself, I was "lucky-allied" (kinda getting with presents relation to close and then give more for alliance) with Torians. Their answer was in diplo room like "well, I am your ally so I am supposed to talk to you" - that's like not very good alliance. With Altarians I was doing badly, I had to give presents now and then to keep them between Wary-Friendly. My morality was just over evil (32 neutral) and that was one thing they didn't like me.
Well, to compare other things happening, I was kinda weakling - my economy was lower that their, my tech was just beginning to increase (actually I hadn't researched battleship where they had overlords - but I managed to get Excaliburs way before them ), overall I was weaker. I began to build Excaliburs on all planets (quite of bunch them) and some combat transports. It seemed like Altarian were going to soon get Excaliburs too so I took step and attacked + invaded their tech center planet . Now, when Torians were allied to both of us, and I was attacker, AND weaker in military numbers (but, it hadnt calculated my most newest Excaliburs which made me almost equal to them in military (but, excaliburs whack other ships, I didnt lost any, they lost all their ships ranging from battleship to overlords, about 200 ships). Yeah, Torians came in war with me, they declared war on Altarians
I don't know why. I just stopped playing for a moment and just sit there. AI is just great in this game. It really suprised me. Actually, I was more than sure they would declare war on me. For military graph window I was bit weaker than them (hehe I had a spike up when my Excaliburs completed) but I hoped Excalibur's quality whacks their outnumbering forces. And actually they did. Last I whacked Torians too
Quite a long story about alliances, wasn't it?

-Sergei