Much has been written about Cambodia's strongman, Prime Minister Hun Sen, who has been in power since 1985. Yet, the history of Hun Sen's early rise to a position of power in the Vietnam-initiated Cambodian revolution after June 1977 remains murky. Relying on Vietnamese and Cambodian archival documents, memoirs and interviews with former veterans of Unit 125 as well as Hun Sen's speeches and personal recollection of his historic journey to Vietnam on 20 June 1977, we make a two-fold argument. First, Hanoi's decision to establish an anti-Pol Pot Cambodian revolution in southern Vietnam to take over Cambodia—after toppling Democratic Kampuchea—was part of Hanoi's strategic plan to handle a double challenge: (1) to avoid being branded as an invader and (2) to establish a capable and friendly regime in Cambodia after the war. This provided an opportunity for a young Khmer Rouge defector, Hun Sen, to change his fortune by quickly earning the Vietnamese military leadership's trust and confidence based on his competence to organize and command the first army unit of the new Cambodian revolution, i.e. Unit 125. Second, as lucky as he was to flee across the heavily militarized border into Vietnam unharmed, Hun Sen's early rise to power is attributed to his survivalist instinct combined with shrewd strategic thinking.