The political cartoon you described, with a man hiding behind a domino while the dominos representing different countries are falling, likely symbolizes the fear of the "domino effect." This concept suggests that if one country fell to communism, neighboring countries would follow suit in a chain reaction, like falling dominos.
Given this context, the most accurate interpretation of the cartoon's message in relation to the U.S. reason for entering Vietnam would be:
**The United States' fear that communism would spread to the rest of Asia.**
This fear was a significant driving force behind U.S. involvement in the Vietnam War, as the government believed that the spread of communism in Vietnam could lead to similar outcomes in surrounding countries.