Which of the following countries fell under communist rule after the americans left?

north korea, china and australia
vietnam, cambodia and laos
vietnam, south korea and the philippines
the peoples republic of china, vietnam and russia

Vietnam, Cambodia, and Laos fell under communist rule after the Americans left. This happened after the Vietnam war. This is known in history as the "Domino Theory," where is one nation fell to communism, the rest of the region and its neighboring countries were likely to follow. Vietnam was the first Communist region in South East Asia, until Cambodia and Laos followed in the year 1975.