It's hard to say when our decline started.
In terms of domestic tranquility, global power, and economic prosperity, I think we peaked in the early 1960s. The decline began around 1965, though nobody knew it at the time.
The occasional bright spots (e.g., the moon landing, the fall of Communism) blinded us to our inner decay.
Reagan gave us some brief economic improvements and foreign victories, like some of the better late Roman emperors, but he failed to stem the inner rot.
Trump only slowed the decline. This country was too far gone by the time he took office.
Besides unleashing inflation, because trade deficits before had to be paid for in gold, international trade imbalances had to remain small because the importer's currency had to fall and the exporter's had to rise. In 1971, Nixon enabled unlimited trade deficits, with the dollar propped up by higher interest rates. This also stiffed our biggest importers, the oil states. Not surprising we got embargoed and had to suddenly pay much more for oil.
In addition, Congress under the Constitution controlled the currency by setting the exchange rate in gold. Now the Federal Reserve controls the value of the currency by setting interest rates, not for the nation's good, but for the banks'.
Nixon also created the alphabet agencies that began an immediate assault on US heavy industry, making them spend billions on pollution controls while at the same time leaving the US exposed to unlimited cheap imports. US heavy industry began to collapse, with steel plant after steel plant and almost all of our shipyards closing.
LBJ........