The box is vacuum-sealed and contains the original treaty between, well, whichever factions in your game are best suited to a Cold War situation. Summer and Winter, or the mortal world and the NeverNever, or Black Court and Fomor, whoever. Opening the box breaks the pristine conditions inside and the treaty crumbles to dust. It's a bit like Schrodinger's Cat - as long as both sides knew the treaty was in the box, they adhered to it even though they could not actually refer to the treaty. Now that's gone, it's war.