It's actually perfectly straigh-forward. Tara already had a unit in Anost. Therefore they held the field (defender).
The Suville units were set to aggressive and were the attackers. Suville and Tara are at neutral status. Sit back and wait...
Aggressive only triggers a battle when neutral and you are the attacker afaik. I still have to discover whether murderous will attack even allies and federation...
I don't think the aggressive settings were necessary, because of the Darkan unit.
The Tarans control the field as you described. Then, the battle is triggered by the presence of the Darkan unit. Because of the CE/Tara alliance, CE joins the defenders.
The Suville units are marked as "freshly arrived", with at least one unit on aggressive, bringing them into the battle. Since they are neutral with everyone involved in the battle, will prefer the attacker side. This puts Suville and Darka on the same side of the field.
If memory serves (because checking the combat script is a nightmare), had Suville had a single unit in the region that wasn't freshly arrived they would have been on the side of CE/Tara. Without the aggressive settings, I believe they would have sat out of the battle.
