Own Force: Fremont + HQ and three recently outsorced Divisions. I mean, I divested them in the planning phase from Fremont. All units had already been at Fort Smith region.
Enemy Force: Prices Division
No unit entered that region during the turn.
My force was to retreat under defensiv posture/avoid battle over the river north to Fort Smith.
Enemy force was also set to defensiv posture. Yet, a battle occured on the dirst day showing both forces been defensiv.
Military Control: 100% Confed (I am the Union player)
As far as I understand the rules (having been changed in some patch, so that the manual isn't up to date), a battle will automatically occur, if a unit ENTERS a region not under own military command if enemy troops are present which are not in a passive posture (which might avoid the battle to happen). This rule should not apply to forces already present in a region not under own command. Which worked during previous turns, where my force waited at Fort Smith under the same conditions. In fact, about a month earlier one of my divisions entered the region as a reinforcement - and no battle occured.
So anybody can tell my, what happened here? As you see in the screeny, no side flipped toward offensive posture. As I am not the host, I have to ask my pbem-player to forward me the files, if somebody wants to take a closer look.