Okay, I've added a black op "land privateer" unit to vanilla Civ3 many moons ago. It worked fine. You could use these invisible land units with no nationality markings to attack anyone anytime without starting a war. You could even use them to attack foreign cities, and, if you kill all units in that city, the black op unit could take over that city (again, without starting a war, though the other civ you're doing this to would figure out it's you and be quite upset.)
Now in C3C, this same "land privateer" black op unit cannot be used to attack foreign cities or units in those cities directly, unless you are at war with that civilization. If you are at peace, then you can destroy other civs' terrain improvements, attack units outside of cities, capture workers and settlers, and bombard cities. But no attacking friendly cities or units in them. And therefore no taking over AI's cities with them, if you are at peace with the civ in question.
Okay, fine. No problem. In a way, I was glad for this change because: 1) it seemed a bit unreasonable that these units, which are made up of small numbers of highly specialized personnel, could take over a whole city, and 2) the AI seems incapable of grasping the full implications of having hordes of invisible units with no nationality markings at your disposal, especially if you don't want to start a full-scale open war, but want to wage a "silent" or "secret" war.
Here's the problem. I was playing C3C, with the v.1.20 patch installed, and found out that AI civs can use the black ops unit to attack my cities and take them over, whereas I cannot do the same to them. How unfair and annoying! When I try to attack a "friend's" city a popup window doesn't appear saying, "If we do this we will start a war with the ::civname::! Are you SURE you wanna do this?" Instead, nothing happens at all.
The AI sure has figured out how to put these bad boys to work. Now my question is, why can't I?? Any ideas, folks?