Unfortunately, it was not always possible to adhere to Nixon’s pledge. By April 1970, a large buildup in Cambodia of North Vietnamese and Viet Cong had been detected in areas adjacent to South Vietnam. It then became obvious to U.S. military experts that the Communists were planning a major offensive thrust into South Vietnam with their supply bases in Cambodia. Such an operation, if successful, could seriously endanger the Vietnamization program. Thus, on April 30, 1970 Nixon sent U.S. combat troops into Cambodia for the purpose of destroying communist bases and supplies; the Cambodian invasion touched off a new wave of anti-war protest in the United Sates. Press criticism of the Nixon administration was intense, and the U.S. senate passed the Cooper-Church Amendment (June 30, 1970) which would have prohibited U.S. military operation in Cambodia without Congressional approval (but the Amendment never became law because the House refused to approve it.) According to the official explanation, had the U.S. not intervened, writes Kissinger, “Vietnamization and American withdrawal would then come unstuck” Strategically, Cambodia could not be considered a country separate from Vietnam; and Cambodia was not “a moral issue” but “a tactical choice.”