Question

What triggered US President Harry S. Truman to send troops to South Korea

Answer 1

To aid the South Koreans against the the communist North Korea who were invading South Korea

Answer 2

On June 27, 1950, President Harry S. Truman announces that he is ordering U.S. air and naval forces to South Korea to aid the democratic nation in repulsing an invasion by communist North Korea. By May 1951, the communists were pushed back to the 38th parallel, and the battle line remained in that vicinity for the remainder of the war. I hope i helped out