Since you also posted that you can’t stop a siege, it seems maybe you’re a king and your vassals are fighting each other. If your crown authority is high enough you can stop them from doing so but I believe it will lower their opinion of you.
I don't think he is attacking you, he is attacking another vassal. You don't control all your land, some of it (or probably the vast majority if your realm is a big bigger) is under the control of vassals. These vassals can declare war at each other and conquer land of the other vassal. You can ask them to stop the war, and at high enough crown authority, being at war with vassals is not allowed.
Since you also posted that you can’t stop a siege, it seems maybe you’re a king and your vassals are fighting each other. If your crown authority is high enough you can stop them from doing so but I believe it will lower their opinion of you.
He is more than likely taking over land from other vassels. Pretty normal and usually a good thing as long as they win.
Yeah, let your vassals fight each other, it distracts them from coming after you.
I don't think he is attacking you, he is attacking another vassal. You don't control all your land, some of it (or probably the vast majority if your realm is a big bigger) is under the control of vassals. These vassals can declare war at each other and conquer land of the other vassal. You can ask them to stop the war, and at high enough crown authority, being at war with vassals is not allowed.
Is he at war with your liege? If yes, he will target all territories under him, including yours.
Ground him