Final answer:
The United States' main goal in the Truman Doctrine, Marshall Plan, and joining NATO was to prevent the spread of communism by supporting democracy, economic recovery in Europe, and forming military alliances for collective defense against Soviet aggression.
Step-by-step explanation:
The main goal of the United States in developing the Truman Doctrine, the Marshall Plan, and joining NATO was to counteract the spread of communism after World War II. The Truman Doctrine promised support to free peoples resisting communist subjugation, starting with aid to Greece and Turkey. The Marshall Plan aimed at economic recovery of European nations to create stable democracies less susceptible to communist influence. NATO was formed as a collective defense pact against potential Soviet aggression, solidifying alignment among Western democracies. In response to the Soviet Union's expansionist policies and support for communist movements globally, the U.S. implemented a strategy of containment. This strategy was significantly influenced by the ideas of diplomat George F. Kennan, who advised promoting capitalism and democracy while preventing the further spread of communism beyond the 'iron curtain' that divided Europe into spheres of influence. As a result, the United States projected its power and ideology through foreign policy initiatives like the Truman Doctrine and the Marshall Plan and through military alliances like NATO. The Truman Doctrine and Marshall Plan also had economic aspects, like recovery and fostering unity in Europe, which in turn would help maintain the political popularity of non-communist entities. Meanwhile, NATO served as a military mechanism through which containment policy could be enforced, deterring Soviet military advances in Western Europe and elsewhere.