Final answer:
The Truman Doctrine was viewed as a threat by the Soviet Union as it explicitly supported efforts to resist Soviet expansion, marking the formal beginning of Cold War tensions.
Step-by-step explanation:
The Soviet Union saw the Truman Doctrine as a threat to its foreign policy because it pledged to support efforts to resist the expansion of Soviet influence. Announced in 1947, the doctrine stated that the United States would provide political, military, and economic assistance to all democratic nations under threat from external or internal authoritarian forces. It was particularly aimed at containing the spread of communism post-World War II. This was seen as a direct challenge to Soviet interests, especially in Eastern Europe where the USSR sought to extend its influence and protect itself by supporting communist governments and movements. The Truman Doctrine effectively marked the formal declaration of the Cold War, where the U.S. would actively resist and contain Soviet expansionism, contrasting the Soviet Union's aims to spread its ideological influence.