Short answer: You told the game that you are not willing to go to battle to help Luria Nova. That's what Peace means. You don't want to fight then, but you don't want to help them either.
Long answer:
It makes perfect sense, in the context of the actual game mechanics. The game can only go by your declared diplomacy. It cannot interpret your intentions. So you have to learn to speak in game mechanics diplomacy terms, and then set your game mechanics to match your intentions. The game cannot adapt to whatever you think that any particular diplomatic state means. It also can't know that your troops are in the region to help defend unless you tell it with diplomacy. If you want to do that, then set your diplomacy to reflect it. Ally with them.
Being at Peace means you don't want to fight them. It does NOT mean you want to help them in battle. You don't really care that Astrum is attacking Luria Nova. Luria Nova isn't your friend. They're just a casual acquaintance that you've seen around, and don't really want to kill. They are NOT someone you're willing to risk your life to help. Which is why you're not going to join the battle on their side.
It helps if you understand how the game system lines people up for battle.
It works in four easy steps:
First:
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Determine who controls the field. This happens even before figuring out whether or not there will be a battle. Any time you have more than one realm in a region, there is a potential for battle, so we have to do this step. There are about 7 different ways the game does this, and they happen in a very specific order. It starts with "Is anyone running a TO?", then proceeds through things like region owner, stationary or moving, and finishing with "Who has the most troops?". The realm in control of field is automatically set as a Defender.
This is the most difficult step in the process, and has the most special cases. I don't remember the exact order of steps, but it's something like this. As soon as you meet one of the conditions, you have your defender and stop there.
a) Anyone running a TO
b) Region owner in the region and not freshly arrived
c) Non-region-owner in the region and not freshly arrived
d) something
e) something
f) something
g) Who ever has the most troops
There's some special conditions in there about peasant mobs and TOs, but I can't remember it. It's there to prevent peasant mobs from being skirmished as attackers.
In your case, someone is running a TO, so they are automatically in control of the field, and the Defender.
Second:
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Once you have determined the Defender, the game uses things like diplomacy and encounter settings to determine if anyone wants to fight them. If the answer is Yes, then you have a battle. That realm is set as the Attacker. If the answer is No, then no battle.
In your case the region owner is there, who is of course at war with the TO force. Now you have someone who wants to attack. You have a defender and an attacker, so you have a battle.
Third:
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The next step is to pick out anyone who wants to help defend the defender. This means anyone who is Allied or Federated with the defender. And *that's it*. There is no "I'm at war with the attacker" check. You're allied to the Defender so you help, or you're not and you don't. Period. (Note that this step does NOT include an "I'm allied with the attacker" check. If you're allied with the Defender, you line up with them. (Unless you're on Murderous. In that case you're an attacker. (Don't you love exceptions?)))
Fourth:
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The last step is to grab anyone who wants to attack any of the defenders, and line them up with the attackers. This includes anyone allied or federated with the attackers. There is a check in this step to prevent lining up on the Attacker side if you are Allied or Federated with any of the defenders. This is where you get things like "X is confused, and stays out of the battle" or "X has friends on both sides, and stays out of the battle".
That's it. Four easy steps to figuring out who will line up where in a battle. It provides a complete, logical explanation for things like lining up with the defender against your ally who is attacking. (I'm looking at you, Strombran! Grr...)
There is no final step of going back and adding anyone who wants to fight the Attackers onto the Defender side. If we did, then you'd also have to add another Attacker check, but then another Defender Check, etc., etc. We don't do endless recursion.
This sequence is simple and reliable. The reason people get confused is because they run screwed up diplomacy because they don't want to commit to war/alliance, or because they assume that the game knows that they're in Keplerstan in order to assist Evilstani in fighting against Lightistan. "We're at Peace with Evilstani, and Neutral with Lightistan, so why wouldn't we help Evilstani attack those Lightistan jerks?"
