Harry S. Truman is known for becoming president of the US after the death of the legendary president and US leader during WWII, Franklin D. Roosevelt. He was known to the public as a WWI veteran, which certainly helped raise his public opinion and rating numbers. His presidency marked the slow beginning of the Cold War, a period which lasted for decades, resulting in the ultimate end of the Soviet Union.
Harry Truman is known for implementing the Marshall Plan, one of the most expensive and ambitious reconstruction and finance programs in the history of the world. The Marshall Plan resulted in the complete rebuild of war-ravaged Europe, bringing dozens of countries from the north and west parts of the continent into the US sphere of influence, which eventually saved those countries from coming under the influence of the USSR.