To unlock 'To Protect And To Serve' you need to meet Nero in the single player campaign.
This takes place midway through 'The Beginning' (Chapter 1). After clearing the Barbarian Horde off the bridge, you will open the doors and progress into the Palace. It is at this point you will meet Nero, The Emperor, and the achievement will unlock.
Can be achieved on any difficulty and is story related and can't be missed.