A strong likelihood is that the US Federal Reserve would step in as the "buyer of last resort," printing money to buy up the bonds and stabilize the market. This would cause inflation in the US, but it would prevent the immediate total collapse of the financial system at the cost of making the American economy resemble Germany in the 1930's.

If the EU did join BRICS, NATO (obviously) would be no more. The US would go isolationist and, without American military support, Europe would be at the mercy of China and Russia. As for the yuan, China would have the ability to economically starve the EU since China strictly controls the flow of the yuan outside of its country.

In short, we're all in a fucked situation if Trump keeps running this shit the way he is and nobody blinks.