The primary evidence for the domino theory is the spread of communist rule in three Southeast Asian countries in 1975, following the communist takeover of Vietnam: South Vietnam (by the Viet Cong), Laos (by the Pathet Lao), and Cambodia (by the Khmer Rouge). the theory that a political event in one country will cause similar events in neighboring countries, like a falling domino causing an entire row of upended dominoes to fall.
