Many news media outlets have said: "NATO has been the world's most successful military alliance, period." But what does NATO really protect. To answer this lets look at the two top layers of the US Empire. That includes the US itself, the main military entity and the top military country in the world. The US runs the world. No one country dare try to oppose this massive empire. We only have to look at ISIS(Islamic State) to see what happens to a nation that goes head to head with the US.
But the US is not alone. The second tier to the empire is Europe. Thanks to both NATO and the European Union (EU) most of Europe today is just one country. These countries, being the most advanced behind the US, share in the spoils of the great and mighty US empire. That means they must also take part in many US military adventures, such as the war against ISIS. When the US leader, such as Trump, calls for a coalition or united effort by NATO, he knows he can count on NATO, because they are all part of the empire and they are subjects of the US Empire.
One thing that comes out of booth the EU and NATO is that no country can do things differently than the empire. The US and Europe are corporate states that always protect the interest in the corporate ruling class. No one dares step out of line. For example when Greece elected the Syriza government so that people could get relief from the austerity measures the EU inflicted on them, that government quickly learned that they are not a separate government and they can't just change the rules just because there people are sick of it. They had to go along with the austerity measures because they are part of the EU and that means they are not sovereign. No country in the world today is really sovereign, unless they are being threatened such as North (Democratic People's Republic of) Korea is.
So NATO is a real success as a part of the new world order, US-Europe empire. NATO and the EU prevent any changes. They prevent any kind of experiment with any kind of new form of government. They also keep the working class in step. Unions are losing strength and no real pro-worker country has any real chance of coming to power much less any chance of survival. This is what today's corporate rulers want and that is what they are likely to keep getting as long as this empire holds.
Next to the US and the European tier comes the bottom tier—all the third world countries that are expected to know their place—at the bottom. Then there are such countries as North Korea and ISIS which have been put on the path to total destruction by the two top tiers of the empire. and in North Korea they will have modest help from South Korea. Trump sees himself as the ruler of the US empire—a mighty empire that intends to rule over all other countries. NATO is the enforcer to that empire. And Trump is now learning that he needs NATO to help enforce his control over planet Earth and all his subjects.