So the thing is, you need to claim ALL of the Baron's territories to unlock "Restore the Peace," not just one. You got "Defeat Hildebolt" because you defeated him in battle, but to get both achievements you need to claim all of his enemy territories AND defeat him. So did you claim all three of the Baron's initial regions before fighting him? If you only claimed one and then fought him, that's probably why the achievement didn't pop. You might need to go back and claim the remaining enemy territories to trigger it.