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.
I'm sorry, but by the logic of your statements, we should be fighting Astrum, because our declared diplomacy is WAR. Not neutrality, where I could see some diplomacy shenanigans showing up. !@#$ING WAR. It isn't a matter of intent, by simple logic, we should be fighting a realm we are at war with. It isn't defending an a takeover, it's fighting a warring realm that
happens to be attacking forces in a region where a takeover is taking place. The only reason your explanation could make any kind of sense is through the extremely convoluted mess that took you several paragraphs to explain. And even then, it only makes sense if you've had to work around the whole archaic system for around a decade. The whole thing is confusing even for relatively experienced players.
