From Wikapedia http://en.wikipedia.org/wiki/Gerald_Ford
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American offensive operations against North Vietnam had ended with the Paris Peace Accords, signed on January 27, 1973. The accords declared a cease fire across both North and South Vietnam, and required the release of American prisoners of war. The agreement guaranteed the territorial integrity of Vietnam and, like the Geneva Conference of 1954, called for national elections in the North and South. The Paris Peace Accords stipulated a sixty-day period for the total withdrawal of U.S. forces.
The accords had been negotiated by United States National Security Advisor Kissinger and North Vietnamese politburo member Le Duc Tho. South Vietnamese President Nguyen Van Thieu was not involved in the final negotiations, and publicly criticized the proposed agreement.
However, anti-war pressures within the United States forced Nixon and Kissinger to pressure Thieu to sign the agreement and enable the withdrawal of American forces. In multiple letters to the South Vietnamese president, Nixon had promised that the United States would defend his government, should the North Vietnamese violate the accords.
In December 1974, months after Ford took office, North Vietnamese forces invaded the province of Phuoc Long. General Trần Văn Trà sought to gauge any South Vietnamese or American response to the invasion, as well as to solve logistical issues before proceeding with the invasion.
As North Vietnamese forces advanced, Ford requested aid for South Vietnam in a $522 million aid package. The funds had been promised by the Nixon administration, but Congress voted against the proposal by a wide margin.
Senator Jacob Javits offered "...large sums for evacuation, but not one nickel for military aid". President Thieu resigned on April 21, 1975, publicly blaming the lack of support from the United States for the fall of his country. Two days later, on April 23, Ford gave a speech at Tulane University. In that speech, he announced that the Vietnam War was over "...as far as America is concerned". The announcement was met with thunderous applause.
The bloodbath in Cambodia, Laos, and Vietnam that had been underway by the Communists for many years now went into high gear, resulting in millions dead and millions more fleeing the country by boat to avoid ethnic cleansing an genocide.
Cause and effect - The anti war movement succeeded, and the US withdrew and denied support to Vietnam, Laos, and Cambodia. If we had done the same thing in Europe, Korea, or Taiwan they would have also been conquered by Communism