So I am at a point in my game (turn 222) where I've created an alliance with 3 AIs, and want to defeat an AI who is not in my alliance, as my forces obviously outnumber and overpower them. The problem is that because the AI isn't that smart when it comes to alliances, the AI that I am at war with will request a truce with my alliance and the AI I am allied with will automatically accept this truce. I am forced to break off from the alliance and risk angering my now potent ex-alliance. What do? 

Never take AI alliances.  They're a trap.  They don't want to win with you; they just want a way to screw you over when you're about to win...even though that means they technically win too.  Never think of the AI as potential friends; despite their thinly veiled attempts to pretend otherwise, ALL of the AI are out to stop you...and only you.  They will repeatedly truce your weakened enemies just to save each other, drag you into wars you don't want just to get you killed, accept truces from enemies that you are trouncing, but are superior to them, that leave you paying absurd tributes that break your bank and never lift a finger when you actually need any kind of aid.  Alliances with the AI are 100% useless and do, in fact, make the game harder to win because they actively sabotage you from the inside.  The AI is spiteful to the point of self-destructiveness and an alliance helps only them, not you.

Perhaps this will change if the word of "AI personalities" takes off, but for now, Alliances are always the wrong answer.

I guess the robots always win then. Thanks

Very annoying indeed.

In previous versions, I can pay a lot to build a large alliance, and win the game overwhelmingly. Perhaps this is the response from the dev team?