This is the 10663rd most frequent Korean word.
군사분계선을
"Military Demarcation Line."
군사분계선을 here refers to the military demarcation line, used as the object of the verb 넘지 않으려고 (try not to cross), showcasing its role as a geographical boundary.
그 여행객은 군사분계선을 넘지 않으려고 신중히 움직였다.
The traveler moved cautiously to avoid crossing the military demarcation line.
군사분계선을 is used to describe the location along which the soldiers were guarding, specifying its function as a dividing line.
군사분계선을 따라 병사들이 경계하고 있었다.
Soldiers were standing guard along the military demarcation line.
군사분계선을 serves here to indicate the subject of a visual or observational activity, its cultural and geopolitical significance in this context.