Trump is making a bad situation worse with his penny pinching ways to showboat to his supporters here in the USA. He was speaking out of his ass during the campaign, but he should know better by now than to continue with that sort of pandering to the lowest common denominator.
There is no other alliance more critical to the safety of the US than NATO and keeping the peace in that region of the world is infinitely cheaper for the American taxpayer than the cost of coming to their military rescue for a third time.
However, the ramifications go well beyond the unimaginable destructiveness of another world war.
Simply giving the impression to our European allies that the USA cannot be counted on for support weakens the hand of Europe when dealing with Russia on economic and political policies as well. A USA less than fully committed to Europe's defense will draw them further into Russia's orbit of influence. That will be unfortunate as our European bases are very important to the projection of our military power around the globe.
Just don't act like the USA is blameless in where we are today.