The trouble is, the world's economy is a fragile beast, and confidence is what is keeping the balls up in the air.
Think of those big economies of South Korea and Japan: they both rely on American presence to keep them in a state of equilibrium with the giant China. If America suddenly turns out to be unreliable as an ally, how long does anyone imagine it's going to take before those nuclear weapons become far more widely spead across the planet? If anything, it gives North Korea ever more jutification for staying nuke: when you become alone and surrounded by foes, you better have some muscle to flex. NK and China may have some things in common, but I don't believe for a moment NK wants to be part of China; if it did, it could have done that without a shot years ago.
Should South Korea suddenly find itself without an American umbrella, how long do you reckon it would take for the north to walk right in, sit right down and make itself at home? An unreliable, isolationist US could be exactly what NK is patiently waiting to happen. That southern waltz would make them immensely rich and with the military might, as a reunited land, to remain safe.
Turning yourself into an unreliable person or country has never done anyone any good. Kurds may be living and fighting in some - to the US - shithole, but don't forget that news spreads fast, and other, more prosperous allies also think about relationships and their relative values in the light of present revelations of perfidious close friends...
Iran learned that lesson decades ago. Does anyone, apart from Israel itself, believe Israel has some divine right to nukes? Really? Gobbling land as it already does?
Isolationism may strike Trump as a clever move, but were it not for the fact that using lower-wage economies to produce US company products has made those products marketable throughout the world, the US would be a far smaller economy than it currently is. If you insist on paying yourselves more than others do, don't be surprised when others can't afford your goods, and you eventually discover that neither can you.
