Final answer:
Truman ordered an invasion of North Korea to stop its attempt to unify Korea under Communism, uphold U.S. credibility against Communism, and prevent a domino effect in the region. The intervention was backed by the UN and led to a stalemate, resulting in an armistice and a divided Korea.
Step-by-step explanation:
President Truman ordered an invasion of North Korea in response to the North Korean army's assault on South Korea with the intention to reunify the Korean peninsula under Communist rule. Truman's decision to intervene was driven by the desire to uphold the credibility of the United States' commitment to countering global Communism, especially after the Communist takeover of neighboring China. The rapid developments in East Asia heightened fears of the Domino Theory, suggesting that a Communist takeover in South Korea could influence the region and potentially embolden the Soviet Union's activities in Europe.
Furthermore, the intervention was supported by the United Nations, with forces from several nations joining the effort under the command of U.S. General Douglas MacArthur. The outcome of Truman's decision saw initial success in repelling the North Koreans and recapturing territory; however, it eventually resulted in a stalemate near the 38th parallel and led to an armistice that restored the pre-war status quo of a divided Korea.