Answer:
The Truman doctrine was the idea that the United States foreign policy should mainly focus on stopping the spread of communism from one country to the next.
Step-by-step explanation:
This doctrine was very important in determining the world events in the second half of the 20th century, because it established the role of the United States as an adversary of the Soviet Union, and thus led to the phenomenon known as the Cold War.
The Korean War for example, was a direct consequence of the Truman Doctrine. The Vietnam War can also be traced back to it even if Truman was not the president anymore when the United States got involved in it.