Both the interior and external situation deteriorated for Sihanouk. In March 1970, when Sihanouk was on a trip to the Soviet Union, he was deposed in a bloodless coup by military chief Lon Nol, supported by Sihanouk's cousin Prince Sisowath (Siri) Matak. The Lon Nol regime, backed by substantial support of the USA, which was worrying about the possibility of communization of Cambodia. The anti-communist government was established in Cambodia, which brought the end of the old political structure and the beginning of the civil war in Cambodia.

The new government began a systematic assault against ethnic Vietnamese in Cambodia to carry out the war against South Vietnam, in particular endeavoring to root out the thousands of Viet Congs and North Vietnamese troops that had based themselves in eastern Cambodia. One result of this action, with unintended but disastrous future consequences, was that Vietnamese Communists and their Cambodian counterparts were driven even deeper into the Cambodian countryside, from where they were in an even better position to de-stabilize the Lon Nol government. A further consequence was that many thousands of Cambodian peasants fled the countryside, heading for the relative safety of Phnom Penh and other urban centers.

Sihanouk expelled by Lon Nol stayed mostly in China, some times in North Korea. After he regained the power, thanking for their cordial hospitality, he designated two streets as Mao Ze Dong Street and Kim Il-sung the Generalissimo street in Phnom Penh. And Pol Pot, who will be discussed later, also had been to North Korea. It reflected that there were some intimate relations between two countries.