The US was at its most powerful at the end of WWII. Its GDP was more than rest of the world combined. Military and industrial capacity - unmatched. And the only power with nuclear weapons. At the end of the war they were barely scratched compared to the total devastation in Europe and Asia.

USA could have literally taken over the world with no one to oppose them.

So ... especially after the first oil shock - you could view US in a state of relative decline. With a short blip during the 90s when they were reaping the spoils of winning the cold war.