A Line Held (and A Line Crossed)
If you missed either of these achievements during your playthrough goto mission select and choose Chapter 13, and proceed to the part where you must run and find your second team mate, here you will see a cut scene after which you have a choice to make.
A Line Held
For this achievement aim your weapon into the air and fire, this lets the local civilians escape, proceed on through the camp and avoid killing anyone as you go and this achievement will unlock.
Video Shows Both Achievements that are unlockable at this pointhttp://game-play.tv/video/H2OWB95BHM28/Spec-Ops-The-Line-A-L...