What is the reason for the decline of the United States?

Others have given much more detailed answers, but I think America peaked in the 1950s and 1960s with Eisenhower and JFK and started to decline once the 1973 oil crisis, Vietnam war and Nixon scandals hit. As many others have pointed out, Reagan has been among the biggest factors in America’s downfall. When you keep cutting taxes for the rich, its obvious the rich will keep getting richer, at the expense of the lower and middle classes. The thing is these days Republicans / conservatives will say ‘lets not help poor people’ or ‘lets not help immigrants’ because they deserve to be poor. Let the rich keep their money cause they earned it. But how much money does the rich really need? Gates and Bezos and Zuckerberg have over 50 billion dollars. That money could have gone to improving the education system, help fund universal healthcare, a better social saftey net, etc. The highest marginal tax rate of the 1950s was 90%. Now its a measly 35%.
Oh, and the military industrial complex is the other thing. The US defense budget is almost $700b now. It dwarfs all other discretionary spending. Since when did the US have to be the police force of the world? That military budget has also taken away money that should have gone to improving healthcare and education.
Somewhere along the way, Americans lost their way. they became too greedy, too selfish and unable to care about other people. today you have the most polarizing environment where they would sooner kill themselves than help the opposing party. Yes, America needs to be ‘great again’, but the answer is not electing Donald Trump.