You will earn this achievement after completing the second level of the campaign.
After waking up near both Hesh and Riley, you'll need to follow them in order to get back onto the streets. Meet up with Two-One and then continue your scout mission. Eventually you'll come across a resistance party of about five or six enemies. Wait until Riley attacks the first one, then start firing. Afterwards, you'll walk a couple of steps and turn the corner to see the enemy executing civilians. You'll have a brief fight with them, and then another, longer fight down another open area. When you reach the end of the area, and end up near a large gate you'll have to pick up a launcher and down two helicopters,
Once that's done, the gate will open, and you will get to the base where Elias is. And finally, after a walk and talk with him, you'll find out some intriguing information and following a fade to black, the achievement is yours.