Sov + champions take enemy city. Enemy sov comes roaring in with an army that can pummel my sov. I stay in the city, he kills me. No problem.  I think I'll sacrifice the city and fall back to get reinforcements from nearby city. As I leave the city undefended, the enemy sov does the ridiculous - he goes around my empty city.  Yes, the AI decides that it's better to wage war in a Zone of Permanent Irrevocable Death than to grab the city, benefit (?) from the defensive structures and follow through to have me fighting him in his turf.

I think it's a result of the AI targetting being too "sticky." I've watched sovs chase CR4 bears all the way to world's end, wasting tens of valuable turns. I'll take a look at the AI scripts this weekend, but I've never written an AI routine before and really don't know what I'm doing.