I was just watching the documentary, The Last Days in Vietnam, and I wondered why did President Ford let South Vietnam fall while we have protected South Korea?
The framing of this question is problematic. Saying, 'why did President Ford let South Vietnam fall' assumes that it was within his power to prevent it. As to why the US did not send troops to try and prevent it, there are several reasons:
-The war had been ongoing for more than a decade.
-some 600,000 allied troops (and ~60,000 US deaths) and the largest air campaign had failed to end it.
-the 1973 War Powers Act placed restrictions on executive power.
-Ford did try to increase aid to the RVN just weeks before it fell, Congress rejected the proposal, though it would have not prevented the fall of Saigon.
-the international dynamics that underlay Kennedy and LBJ's decisions to support the RVN (against the expansion of Chinese influence) were gone. The Sino-Soviet split and Detente had initiated US-China relations years earlier.
-international & US public opinion were against it. At the beginning, Washington reasoned its credibility was tied up in protecting the RVN. But 1975, its credibility was being harmed through its support of the RVN. Eisenhower, JFK, and LBJ had all calibrated their war policy with domestic politics in mind. Re-igniting large-scale US participation was no longer politically advantageous.
South Korea on the other hand has been generally stable since the armistice in 1953. US troops were kept there mainly to prevent the outbreak of another war (each Korea continued to launch cross-DMZ raids). And more recently, it's been more an issue of containing the DPRK/North Korea, maintaining good relations with the ROK, and that there's nowhere else to station those 20,000 US troops. In essence, supporting the ROK is less costly politically and serves a strategic purpose in NE Asia. The idea that Ford could have prevented the fall of South Vietnam, minus a large-scale deployment of troops, is not accurate.
Follow up: I've been told that generals and other officials of both South Vietnam and North Vietnam were amazed by how quickly South Vietnam fell. Is this true? What cause South Vietnam to fall so quickly?