Certainly President Kennedy's much-lamented Alliance for Progress was "in part a victim of Vietnam, in part of historical diffidence," as pointed out in your thoughtful editorial. I submit, however, that it was the awful fiasco of the Bay of Pigs that had the most direct and devastating consequences for the alliance.
In the wake of that incredible event, a triumphant Fidel Castro boasted that he would export his revolution to the rest of Latin America. Official Washington panicked. I know, because I was there as a country desk officer in the State Department. Overnight "No More Cubas" became--and remains--the watchword of our policy for Latin America.
The Kennedy Administration, which had eagerly cultivated the "democratic left" throughout Latin America in order to promote the Alliance for Progress, suddenly had a new priority. Our military aid program for Latin America was quickly reoriented to the new concept of counterinsurgency (meaning, don't bother to look abroad for your enemies when you can find them right at home), and its budget was drastically increased.
It did not take the Latin American military long to realize that they had become indispensable in our new scheme of things, and they proceeded to overthrow or intimidate a number of the democratic governments that Kennedy had hoped would adopt the alliance's economic and social reforms. One notable casualty was that of President Juan Bosch of the Dominican Republic, a Kennedy favorite.