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 Crossed
For this achievement you need to open fire on the civilians in front of you and proceed on through the camp and this achievement / troph will unlock.
You can reload checkpoint to get the other achievement
Video Shows Both Achievements that are unlockable at this point
http://game-play.tv/video/H2OWB95BHM28/Spec-Ops-The-Line-A-L...