My current situation: My empire is in federation. I've been successfully conquering chunks of Empire A (that is in defensive pact with Empire B) until my enemy has only 2-3 planets left and I've gathered enough unity to open (but not complete) the "Domination" tradition tree:
How do I put such claim? All I can see is the "claim sector" option on the claims screen. Is the fact of me being in federation blocking it? Is the target empire still too big? If so, how can I know that the target is small enough (in 1.x you could see that when you were trying to set the war goals)?