Issuing the truman doctrine, defending south korea, and sending military advisors to vietnam were actions taken by the united states to encourage membership in the united nations promote american business in asia gain additional overseas colonies limit the spread of communism

The correct answer is "limit the spread of communism."

After World War II, the United States and Soviet Union emerged as global superpowers. These two countries had very different ideas on how a country should be run, with the US favoring a capitalist and democratic system while the Soviet Union favored a communist system. In order to stop the Soviet influence from spreading, the American government adopted the policy of containment. The goal of this policy was to limit the spread of communism.

This is why the US gave hundreds of millions of dollars to Turkey and Greece through the Truman Doctrine. This also explains why the US gave assistance to South Korea and South Vietnam, as both places were in jeopardy of being taken over by a communist government/country.