So getting away from internal politics, ultimately it's their country they can do what they want, but they have two policies which are going to effect the world in a very real way.
1. Trump is a Climate Change denier, which is insane in this day and age, in fact he thinks it's a huge conspiracy to keep America down. There is this now infamous tweet.
Madness, climate change is obviously a big problem for some of the countries in our neck of the woods.
2. The pulling back support of NATO, this obviously could be a good thing, however there are some problematic places which can't protect themselves. For instance he has said he sees no point in protecting the Baltic countries, which have a very large and persecuted Russian minority. I can see Russia using this as a reason to annex parts or all of those countries from the EU with humanitarian grounds as an excuse, I can also see a Russian supported separatist movement starting from within as there are a lot of seriously unhappy people with absolutely no future the way things stand.