Sisiutil
All Leader Challenger
Yes, declaring war is the one time you can break a trade agreement before 10 turns is up. And no, there's no penalty related to breaking the trade agreement; the "If you make a trade agreement with a civ can you declare (on that Civ) within the mandatory agreement period (the first 10turns)? If so, is there any penalty for doing that (breaking a trade agreement before the 10 turns)?


One other thing that's kinda-sorta of related: if you make a trade agreement with another civ, and a third civ asks you to stop trading with that civ before 10 turns have passed, then the trade agreement remains in place (though I think Open Borders may be an exception; then again, I've never tested it).