In the main, I think USA, up to this point, have been a good and dependable ally. Their firepower allied to that of other NATO members seems (to me) to have maintained peace in Europe, the US and the wider world for many decades since the second world war.
There have of course, been many questions regarding their involvements in South East Asia, Iraq and Afghanistan but nevertheless, the NATO agreement (Article 5?) has been honoured by other members of NATO in these conflicts. There are many reasons why NATO was set up but set up it was and I see it as a "gentleman's agreement" built on trust and honour (there was nothing other than the "agreement" stopping any NATO member from ducking out should they have wanted to in the past - what would really have happened if a country had pulled out? - I can't see that it would have lead to conflict between NATO and the nation in question). The present US government seem to be devoid of the honour required for the present set up of NATO to work. Trump and his cronies are only looking at the balance sheet and see their contribution to NATO to be a waste of money without realising that: a) Peace in Europe and the wider world benefits the US and b) Sometimes money can be spent not because there is something immediately tangible in return but because it's the right thing to do.
I guess being the worlds super power may give them certain rights but this lot seem to choose to ignore the responsibilities that come with those rights and power. I believe that there is probably a very good case for the US to say that Europe should contribute more to NATO but this is obviously not the way to go about it. Any potential "bad actor" can readily see that Europe is as weak now as it has been in the past 60 years due to the complete lack of statesmanship or political nous offered by the US at this time (notwithstanding their abject incompetence at times).
If the US intends to pull out of NATO then surely the way to do it is to announce that this pull out will be phased to ensure that there is no weakening of the NATO military capability. The US excuse for their potential pulling out is that they cannot fight on two fronts (i.e. Russia/Europe and China) - phased withdrawal would ensure that both of these fronts are covered (and probably be of great benefit to the US arms industry!)
In the past the US was seen as a large friendly dog in a small room with a very waggy tail, right now, the whole world sees them as untrustworthy, dishonourable, self serving clowns with a worrying appetite for far right ideology.