WHEN DID AMERICA START TO DECLINE?

That's a question Andrew Sullivan asked his readers and you can backtrack the discussion starting with this link.

I think our decline set in between 1960-75. One of the most important American myths is that we are the Good Guys but that was initially damaged by the televised scenes of racial discrimination in the South.

The escalation of the Vietnam War revealed to millions of Americans that our political leaders and the top military brass will LIE repeatedly to the American people.

The 60s saw the beginning of the movement conservatives assumption of real influence in the GOP, notably chronicled by Rick Perlstein in Before the Storm. You can get a Cliff Notes version of what Perlstein thinks of the 60s here. The 60s also saw Edith Efron, a Ayn Randian, begin the rightwing campaign against "liberal bias in the media."

In the 70s,  the memo by Lewis Powell was the inspiration for the creation of the GOP Noise Machine which only served to debase our national discourse.  Finally, we had the sad spectacle of an American President resigning in disgrace and the general acknowledgment that the Vietnam War was a terrible mistake.