On 1 May 1970, the U.S. Army commenced a series of operations against North Vietnamese and National Liberation Front (NLF) bases inside Cambodia. The Nixon administration sought to bolster the new pro-U.S. government in Cambodia, which had overthrown neutralist Prince Norodom Sihanouk in March. The coup had destabilized the country and brought about military hostilities with the North Vietnamese after the regime challenged communist bases in Cambodia.
The Cambodian incursion had uncertain military results. Although U.S. and ARVN forces captured ample enemy materiel and disrupted supply lines, North Vietnamese and Viet Cong forces quickly restocked supplies with Soviet and Chinese aid and moved bases further into the interior of Cambodia.
Nixon's decision to send U.S. ground forces into Cambodia, however, whatever its military value, provoked fierce reactions among the U.S. public, which reflected the weariness and disapproval of many Americans with the war in Vietnam. Student antiwar demonstrations escalated to unprecedented proportions, providing startling scenes of social unrest. Although the details of the U.S. invasion plan remain disputed, Nixon apparently hastily limited the scope and duration of U.S. military operations in Cambodia after the alarmed response by the U.S. public.
taken from - Kutler, Stanley I. Encyclopedia of the Vietnam War. New York: Simon & Schuster, 1996. p. 98-99.