JD Vance opposes military aid, NATO membership for Ukraine. He's now Trump's VP pick

I'll answer the "why would it now lead to disaster" part, and by shuffling names, places, & contexts around I believe much of that can be obliquely backported to the same question with respect to runup to the world wars of yore.

USA's multi-decade cultural imperialism and dogged pursuit of economic hegemony led to it imposing itself as a global barometer and gatekeeper (despite being famously incompetent at both). The upshot is that now, even though philosophically it would be wonderful for the US to finally stop enforcing a petrodollar-driven serfdom on so much of the world in the name of "infinite (US economic) expansion", pragmatically speaking a magical and abrupt "pull out" would be wildly irresponsible, much like yanking an arrow out of a wound in the direction from which it entered (causing a Jackson Pollock's worth of collateral damage). That would largely be because the sudden power vacuum would not be able to be filled in time by other countries (who are under-resourced due to aforementioned hegemonic squeezing), leaving not enough people to "not tolerate the intolerant", leading to explosive expansion of the "ignorant bullies masquerading as politicians" brigade (not unlike Hitler's opportunistic power-grab while the politicians who should have been the ballwark against him hubristically sat on their hands). See: https://www.ournationalconversation.org/post/explaining-the-paradox-of-tolerance