Try not to play his game. "Picking fights" is classic wayward script, so that he can create some "moral equivalency" with you and justify his affair ("See? THIS is why I can't be married to you, this right here . . . " -- THAT sort of thing).
Don't engage.
So when he says something childish like "The war is on!" just be calm, shake your head or something, and say "Oh please, you're so dramatic. This is your scheduled time with them. Just drop them off at 10, like we planned (or whatever), and try to have a good time. Look, I gotta go."
And then end it.
If he accuses you of being vindictive, focus on whatever FACT is at hand "Umm, no, I'm trying to go meet my sister, because she's expecting me. What time are you going to drop off the kids to me?" ... or whatever.
