The Front Line (2011) on Netflix

Plot summary

Late in the Korean War, with armistice negotiations underway, the hills along the 38th Parallel serve as bargaining chips, and both armies keep fighting to determine the future dividing line. When the commander of Alligator Company at Aerok Hills is found shot by a Southern bullet, First Lieutenant Kang Eun-pyo of the Counterintelligence Corps is sent to the front to investigate the killing and look for a mole said to be mailing letters from Northern troops into the South. He arrives with Captain Jae-oh, the new commanding officer, and a new soldier, Nam Seong-shik. In the unit Eun-pyo finds lax discipline and soldiers wearing captured enemy uniforms, and he meets his old friend Kim Soo-hyeok, whom he believed dead, now a platoon leader in the company.