Final answer:
The Truman Doctrine aimed to contain Soviet expansion by supporting countries threatened by communist takeovers, notably Greece and Turkey, with financial and military assistance.
Step-by-step explanation:
The major purpose of the Truman Doctrine was A. to contain Soviet expansion. Initiated in the aftermath of World War II, this policy was established by U.S. President Harry S. Truman in 1947. It aimed to provide support for countries that were under the threat of communist insurgencies and, as such, became a fundamental aspect of the Cold War strategy of the United States. Essentially, the Truman Doctrine pledged that the U.S. would offer aid to free peoples resisting subjugation by authoritarian regimes, particularly communist ones. This support included financial assistance, military equipment, and the training of military personnel to ensure these nations, such as Greece and Turkey, could resist the communist influence and maintain their sovereignty.