For all but the first few days of the Korean War, the United Nations provided crucial military help to sustain the Republic of Korea (ROK) in its struggle against armed aggression from the north. Of the combatant UN states, the United States provided by far the vast majority of forces on land, sea and air. However, by the conflict's end there were more ROK ground troops in the battle line than those of all other nations combined. The most senior UN commanders were always Americans, though some flag and general officers from other countries held important subordinate operational posts.

Ultimately, fifteen other UN members sent armed forces to participate in the conflict. First into action, and always providing the greatest total numbers, were the British and other members of the Commonwealth, including Australia, New Zealand, Canada and South Africa. A Royal Navy or Australian light fleet carrier was generally present, along with one or two RN light cruisers and several UK/Commonwealth destroyers and frigates. British and Commonwealth ground and air forces were also numerous and in the fight early, with the land contingent building up to division strength. Royal Marine Commandos provided UN commanders with a distinctive, and extensively used, coastal raiding force. The British and Commonwealth countries, given the size of their contribution, also used their own aircraft, weapons and equipment, unlike the other UN forces, which mostly had U.S. weapons and materiel.

Other nations providing forces included brigade or regimental size ground elements from Turkey, Thailand and the Philippines. Belgium, Columbia, Ethopia, France, Greece and The Netherlands sent battalions and little Luxembourg contributed a company. Some of these countries also were represented by small numbers of warships, often former U.S. Navy vessels, and by aircraft. Denmark, India, Italy, Norway and Sweden had medical units in the combat zone. The contribution of the Japanese, still under Allied occupation when the Korean War began, included many invaluable LSTs and inshore minesweepers, plus a significant contingent of merchant ships, stevedores and other hired support personnel in and around Korea, plus the extensive base system in the Japanese home islands.

The Republic of Korea Navy fought hard from the beginning of North Korea's invasion, scoring some very significant successes against enemy efforts to land troops behind UN lines. ROK minesweepers were active throughout the war, helping open ports and keeping coastal waters clear for logistics and gunnery ships. Through the war, the ROK Navy steadily grew as more personnel were trained. The United States transferred five frigates (PF) to Korea, plus many smaller patrol ships, landing vessels and minecraft. The ROK's ground forces and its small air force were also intensively developed, reaching a point where they could be counted as the equal in many ways to the best from abroad.