I've sunk much time into 1212 over the past month, but I still have not a clue how war weariness works. It seems to go up and down almost at random. Several times now I've had it relatively high ~34% and then a minute later, in the same turn, without having made peace or engaged in diplomacy at all, it's down to 14% or even single digits. Then other times I've declared war on a faction and war weariness decreased.
Specifically, how do I reduce war weariness when it is impossible to make peace with other factions?