Was the Truman Doctrine a success or failure? Why?
The Truman Doctrine stated that the United States would provide political, military, or economic assistance to all democratic nations facing internal or external threats from authoritarian sources. This was enacted as a direct result of Soviet expansionism in Eastern Europe. It was also a result of George Kennan's long telegram, in which he wrote that the Soviet Union could not be trusted. The Truman Doctrine had its successes. The United States successfully intervened in a Greek civil war in order to keep the nation from leaning towards communism, and American money given to conservative candidates proved to prevent leftists from taking over in Italy and France.
However, the Truman Doctrine did have its problems. American backing of pro-Western leadership in South Vietnam propped up a corrupt administration that was hated by the people who lived there. Our support of the pro-Western leadership in Iran would eventually lead to its overthrow by Ayatollah Khomeini in 1979. The Truman Doctrine would be quite expensive to operate since America took sole responsibility for keeping the world free from Communism. American increases in military spending led the Soviet Union to do the same--Stalin was quite paranoid during this time since America used the atomic bomb in Japan in 1945 and did not tell him about its existence. Also, some countries in the developing world used the United States in order to get increased aid in order to stop imagined communist threats.
I tend to think, overall, the Truman Doctrine was a success. The idea that the US would offer military assistance to nations facing a communist threat successfully upheld the policy of containment, and did so without direct US military involvement. I would not consider the Korean or Vietnam Wars as pure Truman Doctrine, as future Presidents chose to pursue containment in a more aggressive manner, and Korea was a UN mandate.
I would say that this was a little bit of each. The Truman Doctrine was an early type of containment. It said that the US would help any country that was fighting against being taken over by the communists. Some countries did manage to avoid this, but others didn't.
The two main success stories, I suppose, were Greece and Turkey. In both cases, they repelled communist insurgencies with some US help. By contrast, you can argue that China represented a failure of this policy. The US wanted Chiang Kai-Shek's Nationalist government to retain power, but Mao and the communists took China in 1949.
So there were some results both ways, which presumably means the policy was partly successful and partly a failure.