They're building a wall. Actually they are building several. This is the main occupation of Daniel's squad, when they're not pulling guard duty or walking extra patrol.
The wall is to be the presence in the future, which the Army is currently. It divides ideology. It prevents the flow of animosity down dusty streets which have seen Hatfield/McCoy-like dedication to the anhiliation of the enemy within.
Doonesbury had a comic about the point which Daniel said was "spot on," in his experience. The CLC (Concerned Local Citizen) wanted the Army to blow up the house where an intelligence target was located. The driver asked why and the reply was, "Someone from his family killed someone from my family."
"How long ago was that?" was the next question.
"About 1,300 years ago."