From a purely military point of view: the US can’t beat farmers in a sustained war, the US vs the rest of the world would be over in under a week and most of that time would just be spent on logistics.

Now double that with the economic mess if the US were cut off from the world economy? They wouldn’t be able to field half their military assets without Chinese resources. Not to mention the absolute gutter morale you would have to know the entire planet has allied against you.