They were examples of US policies designed to curb the spread of communism.
Explanation:
The domino effect or domino theory is a Cold War political term first used publicly by US President Dwight Eisenhower in 1954.
During the Cold War, Western countries, and especially the United States, assumed a sudden territorial expansion of the Soviet Union and communist ideology. Domino theory assumes that in the event of a country falling into "communist hands", all its neighbors fall under its influence and in the short term also become communist. As dominoes, all the countries of that region would become communist and communism would spread uncontrollably.
The Truman Doctrine is a US foreign policy plan to stop the spread of communism by giving Turkey and Greece economic aid.
Marshall plan was the official plan of the United States to rebuild post-war Europe and counter the impact of communism after World War II.
