You missed the declaration of war that was in reports - letterbox as well as in the VID section as a result of the minor vassal getting invaded. I recommend checking said letterbox every turn.
There is no diplomatic mechanic for this event. Because it happens frequently when you play with diplomancy, I prepare for this, though, and if you know it will happen, can sort of cheese it a bit. Because of this, I never lower my border guard to friendly majors that have access to my vassals.
In the turn the AI attacks you client/protectorate, you get a declaration of war, but your border doesn't get touched because the AI is sort of surprised by this, too. I then use the next turn to invade said major - we are at war, after all -, grab me some chunks of his territory with the border patrol and as last action of the turn, Propose Peace back to him. Because relation is still in the 90s, this will be an easy task for your foreign affairs director. It was just a misunderstanding, you know...
In the end you will get a new zone from the newly integrated minor, all his troops as militia for you, some border adjustment from the 1-turn-misunderstanding war, can maybe cut off some of his troops or grab a border town from him.
The logistics hell this game is IS the fun part! - Maerchen, 2020
The good thing is, we have all the information in the reports. The bad thing is, we have all the information. Maerchen, 2020
Came for SE. Will stay for SE.