The Republic of Korea Army (ROKA; ), also known as the ROK Army or South Korean Army, is the army of South Korea, responsible for ground-based warfare. It is the largest of the military branches of the Republic of Korea Armed Forces with 365,000 members . This size is maintained through conscription; All able-bodied South Korean males must complete military service (18 months for the army, auxiliary police, and marines, 20 months for the navy and conscripted firefighters, 21 months for the air force and social service, and 36 months for alternative service) between the ages of 18 and 35. History The Republic of Korea Army traces its lineage back to the Gwangmu Reform, when the Pyŏlgigun was established by Emperor Gojong in 1881. The 1st of every October is celebrated in South Korea as Armed Forces Day. It commemorates the day during the Korean War when units of the ROK Army first crossed the 38th Parallel, thus leading the UN coalition into North Korean territory for the first time. The National Security Guard of South Korea was originally formed out of the ROK Army. This organization was created during the United States-led occupation period from 1945 to 1948. The National Security Guard was initially a reserve unit of the National Police. In addition, some Nationalist Chinese and former soldiers of the Manchukuo Imperial Army also contributed to the force. The National Defense Force was established on January 15, 1946, replacing the United States lead constabulary from 1945. The outbreak of the Korean War caught the ROK forces unprepared, requiring the United Nations to intervene with US-led forces. The South Korean military rapidly developed during the Korean War, suffering enormous casualties and loss of equipment. As the Soviets had armed North Korea, the United States armed and trained the South Korean military throughout the Korean War. Current operational status The Republic of Korea Army is structured to operate in both the mountainous terrain native to the Korean Peninsula (70% mountainous) and in North Korea with its 950,000 strong Korean People's Army Ground Force, two-thirds of which is permanently garrisoned in the frontline near the DMZ. The current administration has initiated a program over the next two decades to design a purely domestic means of self-defense, whereby South Korea would be able to fully counter a North Korean attack. The ROK Army was formerly organized into 3 armies: the First Army