The most infamous episode in Sihanouk's life was his early alliance with the Khmer Rouge, formed because the left-leaning king was angry over American bombing raids in Cambodia associated with the Vietnam War. The Khmer Rouge was a communist guerrilla movement that would take power in 1975 under the leadership of the murderous Pol Pot, and then kill up to 25% of the Cambodian population.
The Khmer Rouge kept King Sihanouk under house arrest until they were ousted by the Vietnamese in 1979. Sihanouk regained the throne in 1993, and spent his latter years trying to make peace among different factions in Cambodia. He abdicated in favor of his son, Norodom Sihamoni, in 2004 due to ill health.
Image of King Sihanouk via Wikipedia.