It does already carry a huge diplomatic hit. The opinion hit isn't that significant but it counts as a "provocation" internally which means the AI is much more likely to respond with war and hostility in the future.I think no one can dispute that stealing land during active peace treaty should be either blocked (low hanging fruit), or as Recursive said ideally count as casus belli to the other side and carry *huge* diplomatic hit (but that system is to be designed and implemented yet)..