I think Donald Trump will bring the USA out of NATO, or at least eliminate the US troop presence in Europe.
When Europe goes to war, the USA gets involved.
With nuclear weapons now widely fielded, any large scale war contains risks of a magnitude and type which never before have existed.
I rather think people have become so accustomed to peace they think not of it and assume it will always been so.
The USA has maintained an alliance with Germany (via NATO) for the last seventy years to ensure peace in Europe, so that the USA never has to be involved in the European war.
The EU States have been reckless with this American aid, and spent almost nothing on their own defense.
Now out of the blue a populist, out of his depth, ascends to the Presidency and the USA acts in ways profoundly against its own interest; and this all comes out of the blue, an utter surprise, to an EU lacking so very much in the way of its military strength.
Thank God the Brits and French held on to their own nuclear weapons.
So, the USA will leave, and Europe will need to arm itself, as it should have done (although Trump would have left anyway, I think : some other reason would have been found).
I hope Europe will arm itself through NATO, and if not NATO, then through the EU; the Germans do not want an army, because of how they feel about their past, and if each nation has its own army outside of the EU, then we are back to where we were before, with a range of nations in Europe, and sooner or later, conflict and war.
States can fight, so sooner or later they do.
Europe needs to federate.
Military unification must however be preceded by political integration, just as economic unification must be preceded by political union.
The EU I think is not unified enough politically to support economic or military union, and so when that unification unravels (as it is not complete and not sound), the military will nationialize – as happened in the Soviet Union – and we will be back to where we started, with a range of nations in Europe and sooner or later, conflict and war.