In addition to the monarchy under Sihanouk, there was Khmer Serei and Khmer Krom left but not communist. There was also anti government generals and the pro Maoism Khmer Rouge which was a Communist Party
B 52's airstrikes were ordered which attacked an area near the South Vietnamese border. This is the border separated by the Mekong River. This was 1970. Thailand and Cambodia also share a non river border.The organization SEATO was involved and also Khmer Serei and Khmer Krom
B 52's came from either South Vietnam or Thailand. The bases were closed in Thailand after mobilizations in that country.
Original: Correction on Cambodia 1960's 1970's Domestic and Foriegn Powers