To quote a post by myself from June 2016:
Trump, it turns out, has consistently argued since at least 1990 that the terms of trade between the United States and its allies unfairly favor the latter because the US subsidizes them to a vast extent by providing for free a crucial public good: regional and global security.
It was a valid argument in its day.
Does it still retain its validity in 2017? Since the USSR went out of existence, the US has underprovided security in some important regions. The emergence of ISIS, the bloody chaos in Iraq and Syria, and the resulting human flow towards Europe are the most recent examples of such underprovision. France and Germany can argue with some justification that the American invasion of Iraq, which they did not support, and the subsequent (entirely predictable) chaos in the region resulted in the rise of ISIS. The bitter fruit must now be harvested by the European Union, unavoidably, by virtue of geography. Any clearing of accounts among the G7 countries will have to take this massive American blunder into consideration. Trump may still ask Japan with some reason to pay the US for keeping peace in the Pacific, but the continental Europeans don’t have much to pay for anymore.
America’s role within NATO is a more narrow question than its role as the global policeman. Trump will probably insist on greater military spending by the EU majors and might get his way, but that would not change the terms of trade between the two unions. It might eventually lead to the creation of a continental military force, initially as part of NATO.