I disagree with you on the date! I think that our zenith was reached and died with the gunshots on Nov 22, 1963. It only took several months for Johnson to begin the downward spiral with his War on Poverty, Great Society, and Civil Rights act. He said it best back then, "With the passage of this bill we will have the n****** voting democrat for 200 years!" That was the beginning!!!!
I think Eisenhower's Administration was a plateau after the zenith of the World War II victory.
I think the election of Kennedy, probably due to election fraud, was the beginning of the downturn as the USA began its descent into decadence. The Kennedy Administration and the adulation of the Kennedys were part of the decay.