NATO was formed to prevent yet another war in Europe. After World War II and with the rise of the Soviet Union, it seemed to make sense to develop a collective effort to deter conflict and to defend like-minded countries in Europe and North America. Rather than developing a series of bilateral agreements with many different countries, the US and its friends in Europe formed a multilateral defensive alliance. The whole "an attack upon one is an attack upon all" is the heart of it.
To be clear, it is a defensive alliance so if the uppity Montenegrins (jeez, Trump is dumb) try to start a war, there is no compulsion for the US or anyone else to fight. Yes, alliances have twin challenges--fears of entrapment and fears of abandonment. That an ally may suck you into a war you do not want to fight or may not help you if war comes (Glenn Snyder, Patricia Weitsman). NATO, unlike Trump's fever dreams, mitigates the first problem--that NATO operates by consensus, so if an ally wants to suck you in, you can say no. And, thanks to the key clause in Article V, "each country responds as each deems necessary," even if you go along with a declaration of Article V, any/all countries can opt out. See the Dave and Steve book. Also, we have plenty of practice at not getting sucked into wars started by members thanks to Greece and Turkey (however, the other members may point to the US sucking them into unwanted wars--Iraq and Afghanistan).*
The first big question is easily answered: is it in American interests to invest in peace in Europe? Hells yeah. WWI and WWII were very expensive enterprises, and war has only gotten more destructive. While Trump thinks of these countries as rivals and competitors, the reality is that European countries and Canada have contributed greatly to American interests via preventing war in Europe--that American firms have a secure market to sell to, to invest in, and so on.
The multilateral nature of the alliance--each country agreeing to the collective defense of all members--has benefits that can be best illustrated by looking at East Asia. Japan and South Korea cannot agree with each other on very much, so instead of an East Asian alliance system, the US had a much harder time and a more expensive effort. The US has to work to separate relationships again and again and again, and the level of ROK/Japanese interoperability is mighty low as a result.
Ok, that's the obvious NATO is a collective effort to defend stuff argument. The second and unintended aspect is that Robert Keohane was right--that once an organization exists, countries will maintain it as it continues to facilitate cooperation. NATO has not just been helpful for the United States in preventing war in Europe, it has also facilitated American interests in other ways. Let's listicle, shall we:
- NATO ended one civil war that the UN and EU failed to manage (Bosnia) and stopped ethnic cleansing (Kosovo) that could have destabilized an entire region.
- Despite my criticisms of NATO conditionality, it is the case that NATO helped the transition of East European countries to democracy by encouraging/developing the civilian control of their various militaries--note that whatever movements being made to autocracy are not being led by anyone's armed forces in the region (Vachudova, Epstein).
- Flying AWACS planes over American cities in the aftermath of 9/11
- NATO has had fleets doing counter-terrorism and counter-piracy work. We don't hear much about them, but they have been pretty effective.
- NATO held the fort in Afghanistan while the US was distracted by its Iraq adventure. The allies were very much doing the US a favor at the costs of more than a thousand lives of their soldiers (many private contractors as well, but their lives don't count) and billions of dollars, Euros and other currencies. That the war was complicated by caveats (see Dave and Steve book) does not mean that allies did not contribute and did not pay for it, yes, even the Germans. That Afghanistan hasn't worked out that great is much less about caveats and allied contributions and much more about American bad decisions and, well, third party counter-insurgency being really hard (Simpson)
- Stopping a mass killing before it happened, and, yes, doing a smidge of regime change in Libya.
- Providing a market for American arms manufacturers. Oh yeah, that whole technical interoperability tends to mean buy American. Nice coincidence, right?
It would be dumb to throw it away, especially when its original purpose--defending against the enemy to the east--is now quite relevant again. But Trump is dumb and speaking of being compromised .... Anyhow, NATO has an instrument of American influence and power. If one tosses it aside, well, it is because one is either too dumb to see it or uninterested in having the US remain influential. Which one is Trump?
* No, 2003 did not involve NATO, but members of NATO faced great pressure to join that effort and many did so.