One of tenants of Mr. Trump's foreign policy is going to be some form of fair-share pay by both NATO and Japan. The latter, under Prime Minister Abe, seems to coincide with a desire by the Japanese to have a stronger military in light of a more aggressive China and North Korea.
NATO was conceived as an alliance that would deter invasion by the Soviet Union into Europe. Failing that, NATO would at least be able to hold the Soviet forces until the US could send its forces from the mainland. When the Soviet Union and Warsaw Pact fell, along with the advent of the European Union, the role of NATO morphed from being strictly a defense pact to a quasi-European Union self defense force.
Most would have thought that with the Soviet Union and Warsaw Pact, NATO would also cease. However, NATO allowed the member nations to scale back their military spending without losing the sense that Europe was still able to protect itself. NATO couldn't be eliminated because none of the member nations were spending enough on their own defense to protect their own countries.
The one exception to this rule, besides the US, was the enigmatic Turkey. Turkey is a secular Muslim (the only one in NATO) that since ancient times provides a strategic buffer between Europe and Asia (the land mass formerly known as Asia Minor). Turkey was a member more because of its geography than any ideological, political and certainly theological leanings. The US wanted to base nuclear weapons as close to the Soviet Union as possible and Turkey provided just such a location.
Turkey has always had different views from the rest of NATO. When coalition forces pounded Saddam Hussein into surrender during DESERT STORM, Turkey and Iraq were still very much cooperating along the Turkish/Iraq border. While the US and West were slamming Syria for their politics, Turkey was slamming Syria for holding up the Anatolia water project (which would allow Turkey to sell water to Syria and Iraq).
Now we come to the recent coup that tried to oust President Erdogan. Erdogan's rhetoric and increasingly anti-West leanings caused the US and NATO to look away during the coup (despite the 60 or so nuclear warheads at Incirlik Air Base being left up for grabs). The coup failed and Erdogan has become even more hardline anti-US, pro-Russia. He has now replaced around 150 Turkish officials assigned to NATO with pro-Russia officials.
If Turkey, which has the largest Army in NATO, and the US under Trump were to back out of NATO it would force Europe to either abandon NATO altogether or radically increase military spending. Compound this scenario with the likelihood that more European governments will become nationalistic, anti-immigrant (read, anti-Muslim) and Europe will start to look more like it did prior to World War I.
Mr. Putin may be amassing his forces along the borders of European not so much out of preparation to start a war but rather to prevent a panicky Europe from starting one. As of this writing, Mr. Trump and Mr. Putin continue to seem to like one another and want to work together. Let us hope that this continues and avoid the potential for World War III.