War is a constant in many RPG’s. I believe that I have done enough war based adventures and campaigns that I might as well share some ideas and open the stage for others to share their thoughts.
At regular intervals, a god takes control of your world's weather. Choose the gods at random each turn of the interval and begin applying their effects subtly. Their power will depend on the strength of those who worship them.
For civilised people, worship of a god actually wanes during that god's month in an effort to reduce their potential destructive effect.