Answer:
Containment of communism
Step-by-step explanation:
After WWII Truman's policy towards communism was to contain and prevent its spread, rather than intervene against the USSR and China, which would have been a fools errand especially after the war. After China "fell" to communism and it seemed that Korea would follow, U.S. policymakers feared that all of Asia would be invaded by communist forces and the U.S. would be threatened by an insurmountable military foe, communist countries allied with the Soviet Union taking over Asia and Europe and coming for the USA next.
Korea was occupied by Japan during WWII, and by 1948 the Soviet Union liberated the northern half, then the United States liberated the southern half. The northern half was communist, the southern half was not. In 1949 the Chinese communist revolution created another communist ally bordering North Korea, and with support from China and the USSR, North Korea invaded the South and captured Seoul.
At the time, the U.S. was deeply concerned over the invasion not only because it was hostile and internationally condemned, but because officials feared the "domino effect" of communism spreading from the USSR across Asia and Europe, as well as rising from South America, and the entire world slowly being taken over by USSR-allied communists who wanted to destroy America.