With that being said, the implementation of the Truman Doctrine still represented a turning point in the history of American interventionism because it laid the foundation for the extent to which the United States would intervene in foreign affairs. After the war, many in the United States continued to view the Soviet Union as an ally. Congress showed little interest in combating "Communism," at least initially (more on that in just a second). George Keenan, a diplomat to Moscow, however, took a very different stand. In his "Long Telegram" written from Moscow in 1946 (so-called because it was a relatively long telegram), Keenan warned that the Soviet Union was not a normal nation but an empire that would never cease is attempting to expand its realm of control. In this telegram, Keenan suggested that the United States could counter this insatiable desire by "containing" communism where it already existed. This telegram, as well as its follow-up, the less-secretive "Article-X," strongly influence American policymakers within the State Department and Truman administration from that point forward. Thus we find Truman calling for American support of anti-communist movements in Greece and Turkey and for the "Marshall Plan," a huge stimulus bill that would help to rebuild Western Europe's economy, as early as 1947. Congress, however, showed little interest in these measures and refused to pass the bills that Truman sought. After the Communist coup in democratic Czechoslovakia in 1948, however, Congress came to side with the Truman administration and agreed to fund both plans (Czechoslovakia certainly existed in the Soviet sphere already, and its Communist prime minister Clement Gottwald was already in power before the "coup," but his actions in 1948, which abolished the liberal democratic system and replaced it with one dominated by his Communist Party, frightened Congress into action). With the threat of a Soviet empire looming over Western Europe, Truman was able to convince Congress to provide military aid to Greece and Turkey, to fund the Marshall Plan, and to support a North Atlantic Treaty Organization aimed at preventing Soviet interference in Western Europe. From then on, the United States remained heavily engaged in world affairs, reaching even into the so-called „third world“ to combat the spread of Soviet influence across the globe.

Now you also asked if this was a good thing, if the „impact“ of American foreign policy helped or hurt the world. This is a very difficult question to answer, even when one skirts objectivity. Certainly the original initiatives of the Truman administration – the three described here, i.e., aid to Greece and Turkey, the Marshall Plan, and the formation of NATO – helped to stabilize Europe and prevent the Soviet Union from dominating the continent. Western Europeans were grateful for these American initiatives throughout the Cold War. In a stark departure from a long history of warfare, the threat that NATO opposed to the Soviet Union and its Warsaw Pact proved enough to prevent Europe from succumbing to another destructive war for the remainder of the twentieth century (in part because of „Mutually Assured Destruction“ or „MAD“ created by nuclear weapons, in which both sides knew that an attack on the other would mean the obliteration of ones own nation). Outside of Europe, however, it is not clear that the United States did much good. Scholar's look back to Truman's involvement in Korea, for example, and argue that Truman may have misidentified a nationalist desire to unite the peninsula with the spread of monolithic communism, a common theme from then on. In Vietnam as well, scholars argue that while American policymakers viewed the Communists as agents of a monolithic movement, in reality the United States faced an opponent that embraced Communism in order to garner support from the Soviet Union in order to fight off what it perceived to be an imperialist power. Indeed, the „myth of the monolith“ became apparent with the Sino-Soviet split of the 1960s, and with the wars that broke out between the Communist-led Cambodia and Vietnam after the United States finally withdrew from Southeast Asia in the 1970s. I know that this only scratches the surface of analysis of American Cold War foreign policy, but it's very difficult to provide the kind of answer that you seek. If you have a question about a specific instance, I would be glad to expand on it further.