well, without knowing and understanding global economic and hegemonic theories, it is hard to explain. i used to spend entire courses teaching on these complex topics. so, no easy solutions explanations are possible. but let's try this folks....
keep in mind that following WWII the US built an series of global structures and processes to support itself and its allies politically (UN, world court), economically (World Bank, IMF, etc.), militarily (NATO, SEATO, CENTO, eventually military bases in over 130 countries), and educationally (fulbrights, student mobility, educating global intellectuals, blah....).
this network of systems benefited the rich nations (called the core countries) more often and more effectively than it did the poorer nations. (often referred to as the periphery) this was the intent and purpose of the system - to safeguard global inequality, which it has done rather well.
it resulted in nearly 70 years of the US being the overwhelming center of global growth and influence and a near total lack of war between major powers (ie. no WWIII). of course, war happened, but usually in the periphery, or, when the core exercised its military powers there. (US in latin america, vietnam, iraq, etc.)
the US has paid more and worked more to keep the system going (as trump often notes on who pays for military etc.) but since the US garners BY FAR the lion's share of the benefits, the benefits to the US far far outweigh the costs to it.
so, what trump is doing by undermining NATO, NAFTA, trade with europe, trade with China, limits on international students to the US, cutting of international education funds, etc. is to corrode and undermine the very complex international systems the US set up after WWII. it is making these systems less desirable to other powerful countries so they will start thinking how to do things WITHOUT the US. that is, the systems that created the current world order are being eroded and other countries will try to replace them with systems of their own, but mostly regionally and not globally. (germany will focus mostly on europe, brazil on LA, china on East Asia, etc.)
as with all change this massive, there will be an increase in political, military, economic, and social chaos and disorder throughout the world. BUT as this increases, the traditional ways of dealing with it (UN, IMF, NATO, etc.) will no longer exist/be diminished and therefore the disorder will be harder to contain.
end result? a more violent and disordered world with more violence, starvation, etc etc etc. and fewer solutions, especially at the global level
WHY? simple enough. trump has no idea how any of this works and he only surrounds himself with people of identical opinions and brain power. (remember tillison's famous "he's a ****ing moron" quip.) trump is a honest to goodness simpleton who does not and cannot grasp that economics are embedded in larger world system. he thinks that in world without these systems, the US will be better since it is stronger than anyone.
he fails to understand that much of US power is directly derived from this US created system.
whew... and China? it is simply one major brick being removed from the system. in the immediate future, the momentum of the systems and habits and remaining systems will carry the world forward. but within a decade, if this continues, the world will be facing fundamental changes, and very unpleasant ones.
TL;DR - trump is directly undermining a 70 year old international economic/political/military/educational system that served US interest more than any other country's interests. more benefits flowed to the US than anyone else in this system. without this system and the support of other rich countries, the US will face an increasing fragmented, disorganized, and chaotic world with increased economic instability and political and military violence.
it will become ever more difficult for the US to control events as its loses the allies it won from having this system.