The first statement best describes the Soviet Union's role in Eisenhower's domino theory. The Soviet Union occupied Eastern European countries and imposed communist rule across the region. The domino theory stated that if one country in a region came under communist control, it would lead to the spread of communism to neighboring countries, just as a falling domino can knock over others. In the context of Eastern Europe, the Soviet Union was seen as the primary force behind the spread of communism, and its actions in Eastern European countries were seen as a major threat to the security and stability of the region and the world. The other statements are not accurate in the context of the domino theory.