President Eisenhower's belief that the fall of one nation to communism would lead to others was what?

President Eisenhower's belief that the fall of one nation to communism would lead to others is known as the "domino theory." This concept suggested that the spread of communism in one country could trigger a chain reaction, causing neighboring countries to also fall under communist influence or control. Eisenhower used this theory to justify U.S. involvement in various conflicts, particularly in Southeast Asia, as a means to contain the spread of communism during the Cold War.