You must implement Military force on this mission, after all Freedom is Precious, so it?s a 50-50 chance that the enemy faction will accept surrender, Besiege one of their settlement, while under-siege the diplomat will not have access on that settlement, if there?s another settlement send your diplomat at that place, if that?s the only settlement, find the Diplomat of that faction, if you can?t find one, before you Besiege their settlement order first your Diplomat to talk to them. Fortunately the Senate knows the meaning of consideration, so if you implement military force, even though the Enemy faction did not accept surrender, The Senate will not Note this mission as a Failure, then afterwards they will just order you to take that settlement. For more details about the Diplomacy checkout the Diplomacy section.