A new Trump presidency will plunge the world into something between WW3 and nuclear annihilation.
Europe will have to dust off its own nukes as a serious deterrent to moscow and majorly increase its security spending and preparations, and Ukraine will have to develop its own (which it is perfectly capable of doing), with unpredictable results. If russia tries its luck outside Ukraine, there will be western troops clashing with russians directly.
China will smell weakness and go full throttle on its regional domination plans, and the US will do squat.
Expect a few assassination attempts/successes and unexplained deaths along the way, on almost every continent.
edit: I don't even want to think about the middle east. Without the US acting as a "moderating force", hell will probably break loose there too.
The US benefited from the slavish dependence of european countries on it, and the good business conditions that came with it. This started to change with the EU and in particular with the introduction of the euro.
I guess that it's time for civilized Europe to become a military powerhouse once again.