Sometimes nothing happens and turns later the message is gone.
*nod*, the AI seems to evaluate "do I go to war" at more than one point: at least 2 as far as I can tell.
First, when "I think I want to go to war with X", during which it ramps up military unit production. Then "really, am I going to attack X", where it notices that it still has a huge power gap, and decides maybe a different plan is better.