Harry Truman was the President of the United States from 1945 until 1953. This was the time during which the Cold War started. In general, then, we can say that what he did in the Cold War was act as leader of the United States as the Cold War started. However, there are a few more specific things that he did that we can mention as well. I will discuss two more minor things that he did and three more major things.
The two “minor” things were approving the plans that led to the Berlin Airlift and participating in the creation of NATO. The Berlin Airlift occurred after the Soviet Union blockaded West Berlin in 1948. Truman ordered the airlift instead of either giving in to Soviet demands or doing something more aggressive that would lead to war. The creation of NATO happened in 1949. It was a major event in the Cold War because it created an alliance of democratic nations that were pledged to defend one another against the Soviets.
The three more major things are the issuing the Truman Doctrine, helping to get the Marshall Plan passed, and leading the United States into the Korean War. The Truman Doctrine was issued in 1947. It stated that the US would give aid to any country that was threatened by communist aggression. By issuing this doctrine, Truman committed the US to opposing the spread of communism. The Marshall Plan gave huge amounts of economic aid to rebuild the countries of Western Europe. The plan helped the United States by strengthening its allies and by causing them to feel indebted to the US. These things made the West stronger and more cohesive in the face of the communist threat. Finally, Truman was president as the US got into the Korean War. When North Korea invaded South Korea, Truman pushed for UN authorization for military action and he committed American troops to the war once the UN authorized it.
In these ways, President Truman was a major figure in the early part of the Cold War.