American Military History, Volume 2 by Richard W. Stewart

With most U.S. combat units slated to leave South Vietnam during 1970 and 1971, time was a critical factor for the success of Vietnam- ization and pacification. Neither program could thrive if South Viet- nam's forces were distracted by enemy offensives launched from bases in Cambodia or Laos. While Abrams' operations temporarily reduced the level of enemy activity in the South, bases outside South Vietnam had been strictly off limits to allied ground forces. This rankled U.S. commanders, who regarded the restriction as a potentially fatal mistake. By harboring enemy forces, command facilities, and logistical depots, the Cambodian and Laotian bases threatened all the progress the allies had made in the South since Tet 1968. To the Nixon administration, Abrams' desire to attack the Communist sanctuaries had the special appeal of gaining more time for Vietnamization and of compensating for the bombing halt over North Vietnam.

Because of the proximity of the Cambodian bases to Saigon, they received first priority. Planning for the cross-border attack occurred at a critical time in Cambodia. In early 1970 Cambodia's neutralist leader, Prince Norodom Sihanouk, was overthrown by his pro-Western De- fense Minister, General Lon Nol. Nol closed the port of Sihanoukville to supplies destined for Communist forces in the border bases and in South Vietnam. He also demanded that Communist forces leave Cam- bodia and accepted the South Vietnamese government's offer to apply pressure against those located near the border. (A year earlier American B–52 bombers had begun in secret to bomb enemy bases in Cambo- dia.) By mid-April 1970 South Vietnamese armored cavalry and ranger units, with no U.S. advisers accompanying them, were mount- ing large-scale operations across the border from III Corps and uncover- ing large caches of enemy supplies and equipment.