During the Cold War period, the United States entered conflicts with Korea and Vietnam to prevent the spread of communism. Some theorists believed that these regions surrounding the Soviet Union had to be protected because gaining control of these areas would lead to world domination. Which of the following theories is MOST closely related to this idea?

Dependency theory
Domino theory
Heartland theory
Rimland theory
I only
III only
IV only
I and II
I, III, and IV