So you need to know when there's going to be a battle? And you don't know who is fighting with or against you? You're one of those people who points the sword towards themselves aren't you? Now don't get offended, I understand that diplomacy is a very complicated thing that is difficult to understand. If you throw unit settings on top of that, its a mess even an ambasador can't handle. So if you've got some time and a few mugs of ale, I can try to help you.
- Rogues are considered at war with everyone.
- Higher relations imply friendlier relations and moving closer to a Federation.
- Lower relations imply a greater dislike and moving closer to Hatred.
- If all of the units on one side are able to evade, there obviously won't be a battle.
Well, diplomacy is quite easy with only two realms. If the realms are at War, a battle is guaranteed. Also, a battle will occur if the realms are neutral and a unit is set to agressive or if the realms are allies/federated and a unit is set to murderous.
Alright, here is where it starts to get tricky, so pay close attention.
First, lets assume that the three realms are all at War or Hatred. A battle is guaranteed, but how are they going to fight? They all want to kill each other, but since a three realm free-for-all is out of the question, it must be 2 vs 1. Now we need to determine who the attackers and defenders will be. 99.999999999999% of the time, units from a realm that just arrived in the region or have just rallied are going to be considered "attackers". The defenders are usually the realm that "had control of the field at the turn change", which basically means whoever was sitting in the region for a turn without being scattered or evading. Everyone labeled and "attacker" will work together to defeat the "defenders" in the upcoming battle.
I know, I know. Your head should stop spinning after a few minutes. In the mean time, here's an example to help clarify the point.
You can also check the Diplomacy Effects page for a step by step explanation.