How did the Truman Doctrine directly leads to the U. S. involvement in Cuba, Korea, and Vietnam?

During Truman's presidency, the Truman Doctrine would result in another conflict in Asia, this time in Korea, as the US government attempted to prevent the unification of Korea under a communist government. Truman's policy would continue to drive American interventions through the 1980s

the answer to your question is,

old stone age.

-mabel < 3