"We've made it abundantly clear that we really aren't going to be the "World Cop" protecting them from the bad guys anymore. That we fully expect them to massively, and rapidly, increase their own defense capabilities and start operating as a strong military power commensurate with their economic strength. That we regarded the historic collective security arrangement under NATO as Europe "free riding" on our coattails. After all, we weren't getting anything out of it.

Well, one of the consequences of forcing Europe to be stronger on their own is that they have more leeway to say "no" to us on things. We benefited from them being dependent on us militarily. We benefited from being the oversized contributor to NATO. It meant that we were the ones that were completely in charge of most security decisions - that if we wanted to engage in military exercises, everyone would have little option but to go along with it.

We've changed that. We get the benefit of spending less on military support for Europe. But the cost of that benefit is that we don't get to be quite as influential in Europe. Turning a loyal supplicant into a militarily strong competitor doesn't seem like it's the most beneficial thing for the U.S. to do....but there you are." - Albaby

While all of this is certainly true and definitely enough to demonstrate why the Trump administration backing away from being the "tentpole" of NATO is utterly stupid, there is also a more subtle reason as well.

Basically, countries with large powerful armies find reasons to use them. That is just basic human nature. Forcing the European countries to increase their military spending to build their nations military might will encourage them to use their military might. This is a bad idea when there are dozens of European countries with centuries worth of bad blood between them. Especially when there is Putin in Russia willing to use information warfare and sabotage to agitate those sore spots between countries.

Having lots of countries in Europe with more powerful armies has never worked out well for the world and actively threatens the security of the U.S.