Take a look at this map:
It's a map of the American river system.
In the 19th century, the only people who could get their goods to market were those who lived near the mighty Mississippi or one of it's many tributories. Grain from the prairies or goods made in Chicago would be driven to the river by wagons, and then floated down by riverboats to the Gulf of Mexico. From there they were put on sea-going vessels that then sailed up the east coast to deliver to Washington D.C., New York and Boston, as well as exporting to Europe.
Even when railroads arrived, they mainly connected mid-western cities with the Mississipi.
The invention of the internal combustion engine in Germany in the 1890's changed everything. Powered by petroleum and diesel, it made trucks and planes viable.
Eisenhower did the rest by building the Interstate highways. Here is a map of US interstate system:
Every town has roads and all roads connect to the interstate system. So goods can flow freely - you just need to truck them.
Think about how much food and wine California produces and how none of it would reach the rest of the USA without diesel-powered trucks. Think about how a state like Arizona had a population of only 138,000 people in 1900, because it was cut off from the river system. It now has over 7 million people because it's connected thanks to the internal combustion engine.
Of course the internal combustion engine relies on oil, which has been pretty cheap in the last 100 years relative to other goods. If that changes...
It's not China that is threatening the American economy and thus American hegemony. It's the price of oil.