The controlling idea is that the USA was responsible for the Berlin wall, because it wanted to see how the Soviet Union would react.
Explanation:
A controlling idea represents the opinion of the author or a character in relation to the topic being addressed in the text. In relation to chapter 5 of "The Dark Game," we can see that the controlling idea is that the USA had a strong interest in maintaining the Berlin wall, as that wall represented a valuable source of information about the Soviet Union, as the US could observe the Soviets' ways of acting and the policies established by them in an isolated region and without popular support.
