I know that things look like they are going to hell "suddenly" with the present...uhhh..."administration", but I have always considered that they started downhill shortly after the assassination of President Kennedy (and I don't think it had anything to do with the cult of personality surrounding Kennedy). I don't know if it was the Johnson administration, the handling of the Viet Nam war, or what. Maybe it was the assassination itself, that a single individual nothing, an apparently "lone gunman", could become important on the world stage and could dramatically influence the course of history. Somehow, being an "indivdual" became more important after that day.
Coincidentally, I wonder if future history books will someday show that the zenith of the US was actually sometime in the 1960's.