If Trump goes ahead with his trade wars and it spirals out of control other countries may move away from the US$ as a reserve currency. The end to the "free loans" enjoyed by the US would lead to higher interest rates. Prices would rapidly rise. .... Political unrest?
The USA has lower foreign reserves than say Indonesia. It is not in a great position to play games with its largest shareholder = China.
The Federal Reserve is currently slowly reversing QE, i.e. gradually putting its massive Treasury Holdings back on the market. Tax cuts mean borrowing to finance government expenditure will increase by a lot in 2018. The Fed has a lot of big auctions of Government debt to undertake.
Any country - e.g.s China, S.Korea - might retaliate in a Trade War by ceasing to place part or all of their new cash (from Trade Surpluses) in US Treasuries and / or disposing of part or all of their existing US Treasury holdings.
The effect at the least is upward pressure on interest rates (everywhere), and at worst a crisis of confidence in the Financial markets that most likely would immediately become world-wide. The US may lose its status as the no.1 Safe Haven.
The world is awash with debt - worse than 2008. Trump is playing with fire. When Goldman Sachs is scared so should we be (no matter what we think of a system where Goldman Sachs has such clout).
CO2 emissions - many poorer countries will not have the money to invest in renewable energy if the world goes into recession and the financial markets can't provide low-interest loans. Clapped out fossil fuel based plant, machinery and transportation will be used until it collapses.
Alas, Paris 2015, I knew thee well.
Too many Doomsday scenarios when looking at the world - risk analysis of them all would produce a result of more than 100%, i.e. at least one or more ooh-nasty must happen.