The diplomatic screen is at the moment a complete, indecipherable mess.I talk to Tarth, and see a bunch of treaties listed on the top of the screen, some with indecipherable numbers beside them. There are also other treaties listed as buttons on the bottom of the screen: when I click on one, it greys out and appears on the top list. This seems to make the treaty "valid", as I had clicked on Peace Treaty after declaring war and the war stopped. However, nothing will make the treaty buttons become available again after clicking them.

I also entered Tarth's territory after getting the non-aggression pact, and attacked their city. The lone soldier inside put up no resistance whatsoever (no battle window) and the city immediately becamed mine ALONG WITH THE SOLDIER. The territory, however, remained Tarthan yellow. This finally got the Tarth to declare war on me. I made a peace treaty with them, and while I could move the soverign out of the city, I couldn't move to any other square because I got a "violating borders" dialog. (Remember, this is all in the same turn). The next turn, the borders turned to my color, and I could move freely.