There is no other nation that has the global footprint that the United States of America so represents, and there isn't any other nation that even holds a real candle to the size, strength, and wealth of America. This signifies that Russia, which is a nation that has a population that is less than 150 million people, while also having a GDP that doesn't even rank in the top ten, is not a real and present danger to America. As for China, while it must be admitted that it is far more populous than America, as well as being second in GDP, it still is years away from being a true threat to American hegemony, of which, China may not be inclined to do much of anything in that area, as China smartly believes that being patient and non-bellicose is the better frame of mind to project.
The main issue as to why the American empire is in decline, really comes down to the fact, that America directs way too many of its resources and its military into areas of this planet, that doesn't really need or necessitate direct American involvement; and by doing these very things, puts America more deeply into debt, while also upsetting country after country that America does flex its muscles and bosses around, as opposed to America representing itself as a bastion of freedom, liberty, meritocracy, and opportunity through good diplomacy.
Regrettably, since America feels the need to get its hands into as many pockets as possible, and to intertwine itself into the affairs of many a foreign nation, it has exposed itself to the type of vulnerabilities that are sure to have the type of unintended repercussions which have yet to be fully realized. Look, it has to be said, that as strong as America is, the fact of the matter is that America has neither the population numbers nor an infinite amount of military might to impose its will upon every other nation in this world, and those that America believes are on its side because of NATO, or other agreements that have been ratified, don't seem to comprehend that history is replete with nation after nation that breaks treaties at opportune moments, which thereby changes everything.
The thing that America does not seem to be fully aware of is that there are those tried and true friends, and then there are those fair-weather friends, or even those that pretend to be our friends but really aren't; so that, when the American empire beings its collapse, it isn't going to just come out of anywhere, but rather the seeds of its own destruction of that empire have been already sowed, and when that harvest comes in, the dismay that Americans will feel will be palpable, along with being quite concerning, of which, unfortunately for Americans, by the time they figure out what is really going on that empire will no longer be gradually dissolving, but will rather be subject to a very sudden and disastrous catastrophe, which will, when the dust settles, put America back to where it formerly was, or perhaps far worse, because payback is an incredibly terrible thing, but what will be, will be – because even a great empire, must in the end, pay in full what they do so fairly owe.