American died when the North won the civil war.
It’s pretty much been all downhill since.
Our political system certainly changed after the Civil War. The 10th Amendment was pushed to the back and federal power expanded beyond restraint. This was not a good development, but one could say that the advances our country made after Reconstruction were without parallel in human history.
IMHO, America after WWII, was clearly at it is apex. We were the greatest nation that ever existed offering more, freedom and opportunity to more people than ever in human history. The real slide began with the change of mores in the 60s. Once we we became decadent, we lost our way.