Final answer:
George F. Kennan influenced the Truman Doctrine, which aimed to contain the spread of communism. Kennan advised Truman to promote capitalism and democracy while constraining Soviet influence, leading to the adoption of the policy of containment.
Step-by-step explanation:
Influential historian George F. Kennan had a significant impact on the formulation of the Truman Doctrine. Kennan viewed the Soviet Union as an aggressor state seeking to expand its influence, and he advised Truman to respond by promoting capitalism and democracy while containing communism. Truman adopted Kennan's advice and implemented the policy of containment, which aimed to restrict Soviet expansion. This policy formed the basis for US foreign policy for several decades.