So how do you get the AI to break an alliance and attack its AI alliance partner?

I don't think that there is any way to break an AI alliance (other than removing one of the AI from the game ;))

Nope. I've broken up an alliance before by allying with one of the races and declaring war on the other, but there seems to be a high probability that the two AIs will stick together. (My military power was simply rediculous when I broke up the Altarians and the Torians, and according to the diplomacy screen the Torians were getting 102% of their income from trade with me.)

you can do this by forceing that empire to attack that empire and then you got a war:).

I hadn't ever thought of that. Interesting.

This has generally been my experience, never managed it yet, what I occasionally have done however is built a few corvettes along the allies trade lanes and gifted them to someone they're at war with. You destroy the allies trade income at least

I did the same thing as as PJ. I allied with the Altarians, who were also allied with the I-League. I then attacked the I-League, and was promptly joined by the Altarians (who were being extra-aggresive in that game, they'd already wiped out the Carinoids, the Scottinglas, the Arceans and the Drengin). As an extra bonus, my friends the Yor gave me a bunch'o'crappy ships.

I have played a game where I was allied with altarian, and altarian where allied with torians and they were the majors AI left. And I was the 3rd in military .

I managed to have the altarians declare war to the torians: I put some offensive ships in Torian sectors. They declare war to me. They destroyed some of my ships but I didn't respond. Then the Altarians came to help me

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That has to be a government statistic!

It's not possible to make to empire of friendly status with another declare war on that empire. It doesn't show up on the diplomacy options screen.

If you get really lucky, a random event will occur between the two parties, breaking the treaty thus allowing you to align with one or the other. Saved my butt once. Sometimes lucky is better than mediocre.