The Allies of Your New Allies

It is not clear to me how the AI whom I have a cease fire and peace treaty with and 80%+ relations can all of the sudden go to war with me.  I suspect the reason the AI did so was I went after one of his allies who was not yet my ally.  But I don't know this for sure as I have not seen it mentioned.

Also, I would love to help my teammates when they are attacked, but when they are attacked by my new friends, I am unable to assist.

Also, In my 3 v 3, one of the enemies (later made my ally) is all that is left.  My original teammates are alive too.  But I am confused as to how to end the game???  Am I supposed to click "Go to war?"