I am allied with the Black Oath and cannot destroy the jammers, they are friendly (green)?

How do I declare war against someone who owns nothing?

https://www.youtube.com/watch?v=IrF9sO94mMo

Ahh, I watched the video again and there is a button in Records -> Reputation to talk to the Ambassador :)

https://www.youtube.com/watch?v=pSg2mvYcNkw

No Luck

I am now invading allies of The Black Oath and they are hostile again ^^

That did it, progress. Good you can use the game mechanics to help yourself.
Really good story, I feel a bit hopeless and betrayed right now...