The Truman Doctrine was shaped by the assumption that the Soviet Union was aggressively seeking to expand its influence by supporting the rise of communist governments across the world. With this assumption in place, United States foreign policy became increasingly interventionist, seeking to contain the further spread of communism around the world. The Truman Doctrine served as a key expression of this new interventionist mindset, as the United States committed itself to granting "political, military and economic assistance to all democratic nations under threat from external or internal authoritarian forces." (see Reference Link courtesy of the US State Department)

One thing you should remember is that this entire policy was launched in the aftermath of World War II, which left much of Europe economically devastated from the impact of the war, even as the Soviet Union gained hegemony in East Europe and the communists emerged the winners of the Chinese Civil War. This was the context in which this approach to foreign policy took place.

After the Second World War, the United States devoted extensive resources to the rebuilding of war-torn countries, with the objective that this investment would help prevent them from falling to communist revolutions. In addition, however, you should also be aware that the Truman Doctrine carried with it military implications as well, as can be seen with the example of the Korean War, with the United States leading an armed intervention after North Korea's attempted invasion of South Korea. When determining how successful the Truman Doctrine might have been, it is these various outcomes that you should strive to assess.