I was speaking in the case that they had already rejected your treaty.
Sure they had the option to betray their allies and lose no lands in the short term, but I would not consider them rejecting it to be the wrong move. Some prefer to fight and stand by their allies rather than turn on them. If you look solely at the realm regions etc, yes then it was a huge mistake, but otherwise it wasn't (or doesn't have to be depending on your priorities and your char's sense of honor etc).
