Actually really started on 1/20/2021.
We had a Good Run.
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.