Sadly i was worried about this loophole.. I have a fix for this but i wanted to see if people would try to use this loophole before i made it..
Anyway, if you want or shall i say if this becomes a problem, then we'll just add another time limit to the "Prepare" feature. Which would make it so if you have prepared for war, you only have *blank* amount of time till it expires and then you have to prepare all over again..
Or we can leave it like it is as someone who goes around and Prepares against all nations would surely create some diplomatic rifts with other nations.