I don't think there IS any threat against the USA. Nobody can invade a whole continent by force. It was only possible in the days of colonialism when Europeans had firearms and the rest of the world did not. I agree that a nuclear attack is a possibility but the only country that is a threat to the US on that score, is Russia. And a nuclear war is not worth fighting because it is a suicide to start one. It is known by all sides.
But the biggest industry in the US is conventional arms manufacturing, and war (or at least tension/cold war) is their business model. They create a market for their products, by war mongering and scaring their own population, plus actually starting wars themselves.
Plus, they need to ensure access to natural resources. Either voluntarily through trade at favourable costs, or involuntarily, or by putting in place a pro US government that lets them trade on their own terms. Much more clever and slick than traditional colonialism, and it doesn't seem as evil either. The US is just like a hungry dragon that constantly need feeding. It needs more resources per inhabitant than any other country.. and sadly the resources are not particularly evenly distributed either.
North Korea has much more reason to fear the USA than the other way around. There are nukes pointed at them from US bases in South Korea.
I think you are right though. China gets hold of what it needs from Africa and South America by simply trading or exchanging expertise for resources. That is the way forward. More fair trade on decent terms. Not exploitation and imperialism.