The Korean War was an escalation of border clashes between two rival Korean regimes, each of which was supported by external powers, with each trying to topple the other through political and guerilla tactics. In a very narrow sense, some may refer to it as a civil war, though many other factors were at play. After failing to strengthen their cause in the free elections held in South Korea during May 1950 and the refusal of South Korea to hold new elections per North Korean demands, the communist North Korean Army moved south on June 25, 1950 to attempt to reunite the Korean peninsula, which had been formally divided since 1948. The conflict was then expanded by the United States and the Soviet Union's involvement as part of the larger Cold War. The main hostilities were during the period from June 25, 1950 until the armistice (ceasefire agreement) was signed on July 27, 1953.

In South Korea, the war is often called 6·25 or 6·25 War (Korean: 6·25 전쟁), from the date of the start of the conflict or, more formally, Hanguk Jeonjaeng (Korean: 한국전쟁; Hanja: 韓國戰爭, literally “Korean War”). In North Korea, while commonly known as the Korean War, it is formally called the Fatherland Liberation War (조국해방전쟁). In the United States, the conflict was officially termed a police action — the Korean Conflict — rather than a war, largely in order to avoid the necessity of a declaration of war by the U.S. Congress. The war is sometimes called The Forgotten War because it is a major conflict of the 20th century that gets far less attention than World War II, which preceded it, and the controversial Vietnam War, which succeeded it. In China, the conflict was known as the War to Resist America and Aid Korea (抗 美 援 朝), but is today commonly called the “Korean War” (朝鮮 戰爭 Chaoxian Zhanzheng, 韓國戰爭 Hanguo Zhanzheng, or simply 韓戰 Hanzhan).