As a student in a graduate seminar on the Korean War, I remember hearing the number of around 15,000 South Korean policemen who died during the war from 1950-53. My research this morning has, unfortunately, been unable to confirm that number. If true, to put it in context, around 35,000 American soldiers died during the war; this would point to the key role that South Korean police served during the war, and the sheer amount of dying that their ranks suffered. Again, however, I can't confirm that first number.

The Korean War was as much a battlefield war with frontline combat as it was a civil war, beset by insurgency and militarized rear echelon areas. While military units engaged in counterinsurgency efforts during the war--as history buffs of counterinsurgency operations elsewhere in the world will know--political and military officials relied heavily on South Korean police, envisioned as contributing to a war effort in a broad and very granular sense. "Anti-bandit operations" was a common term employed by U.S. advisors to describe the responsibilities of Korean National Police. The objectives of National Police were very sweeping indeed: the "protection of life and property," "enforcement of criminal codes," "maintenance of internal security and good order," "police of inland and off-shore waterways", along with other activities like enforcing sanitation, "regulating" refugees (which could have included attempting to filter out suspected saboteurs, a common concern among war refugees coming from the North), and the "guarding of vital UN installations," which could include strategic points like railroads, key buildings or crossroads, etc. South Korean police were responsible for trash collection just as they were for intelligence work, patrolling, and aggressively hunting enemy guerrilla forces. All this for an institution that had a terrible public reputation, dating back to its history as a colonial police force that supported Japanese control of the peninsula.

In other words, Korean National Police had an extremely expansive mandate. They also were involved with some of the worst massacres and human rights atrocities during the war, including the mass killings of suspected communists (the most atrocious of which happened during the South Korean state's initial retreat in the summer 1950, and subsequent reprisals when it recaptured territory).

Another indication of the importance of Korean National Police to the UN war effort is the presence of American advisors, which is my main area of expertise. U.S. advisors are most known for advising and influencing the ROK military, but U.S. military personnel were also dispatched to advise Korean police commands. There are records in the U.S. National Archives of recommendations for medals and other commendations for key South Korean policemen for various acts of service. One recommendation for a Bronze Star written by a U.S. advisor (which the 8th US Army ultimately declined to award) cites a "Captain Ahn" for his professionalism, English fluency, tact, diplomacy, and efforts to improve the National Police's administration and supply system. Another police officer, a "Chief Kim" of the Kyongeang Namdo Province, received commendations for training and retraining 17,000 to 19,000 police officers and leading them in successful counterinsurgency operations against "bandits" in rear areas. Police during the war were advised and influenced by U.S. military personnel to carry out their efforts in service of the war effort. This is a key difference to U.S. advisory efforts to the Korean National Police after the war, where U.S. military personnel began leaving and their duties taken up by civilian experts in criminology and policing.

I hope this answers your question to a degree, and illustrates one single instance where police served a key role in an armed conflict, and constituted a significant portion of the people doing the killing and dying of a war.

Writing this short answer left me quite frustrated--at myself! I went back into my saved records at the U.S. National Archives that covered the Korean National Police and saw that my photographs were wholly incomplete. I must have considered most of the documents to be irrelevant to my research at the time, but now I find myself wondering with what I left out. I am unfamiliar with Korean-language scholarship on the history of the Korean National Police, which undoubtedly could provide far more information that I have outlined here.

Much of the information I am citing was what I personally found at the Archives, which, for someone who may be interested, are located in Record Group 469, under the series title "Records Relating to the Korean National Police, 1953-1959."