It has been said that the USA used Cambodia as a testing ground for the development of bombs.
To use the passive voice phrase, "It has been said", requires responsible writers to cite their sources when making inflammatory statements.
North Vietnam, after invading South Vietnam, controlled vast territories in Cambodia wihich they used at will to attack Americans and South Vietnamese across the border. King Sihanouk allowed the communist Vietnamese this sanctuary as early as 1965. (Lonely Planet Cambodia, Fifth Edition, Pg. 30)
Right or wrong, the bombing was a defensive measure to prevent these aggressors from using , with impunity, Cambodia as an offensive base.
Don't believe all the posts you read in Thailand regarding America because there is a lot of misinformation from former colonists who are jealous of America.
Why do you think there is no mention of the country that supported and supplied the Khymer Rouge with weapons, and even fought a pitched (and losing) battle against liberating Vietnamse troops in early 1979? (Lonely Planet, Pg. 34).
Why? Because they were not Americans.