However, another major factor causing the spread of the war to Cambodia includes the U.S. involvement in the conflict, and minor causes such the ruling of Prince Sihanouk whose governing took an unexpected turn, and the rise of Lon Nol. Prince Sihanouk was overthrown by Marshal Lon Nol in 1970 and he retaliated by joining with Pol Pot, further allowing the Khmer Rouge to become a major player in the civil war. The Khmer Rouge began to defeat Lon Nol's forces on the battlefields. In 1969, a secret bombing campaign ordered by U.S. President Nixon lasted for four year. This campaign was the cause of hundreds and thousands of deaths in Cambodia. Eventually, Cambodia's government lost its American military support when they withdrew troops from Vietnam in 1975. Lon Nol's defeat to the Khmer Rouge led to Pol Pot's rule and the announcement of a Communist state. In power, the Democratic Republic of Kampuchea carried out brutal cleansings where 25 precent of the population died from executions, work exhaustions, illness and starvation. These murders and arrests transformed the party into a terrifying reign of violence that went on until the Vietnamese invasion in January