Donald Trump shocked foreign-policy professionals and observers when he remarked to The New York Times that if he were president, the United States might not come to the defense of an attacked NATO ally that hadn’t fulfilled its “obligation to make payments.” The remark broke with decades of bipartisan commitment to the alliance and, as Jeffrey Goldberg wrote in The Atlantic, aligned well with the interests of Russia, whose ambitions NATO was founded largely to contain. One Republican in Congress openly wondered whether his party’s nominee could be “seemingly so pro-Russia” because of “connections and contracts and things from the past or whatever.”

It’s not unlike Trump to make shocking statements. But these ones stoked particular alarm, not least among America’s allies, about the candidate’s suitability for the United States presidency. So what’s the big deal? What does NATO actually do?

The North Atlantic Treaty Organization was formed—three years, two months, and 10 days after Donald J. Trump was born—to keep peace in post-World War II Europe. But Lord Hastings Ismay, the alliance’s first secretary general and a friend of Winston Churchill, is said to have remarked that the alliance really had three purposes: “to keep the Russians out, the Americans in, and the Germans down.”

The treaty had evolved out of an initiative of the so-called Benelux countries (the vertical stripe of Europe comprising Belgium, the Netherlands, and Luxembourg), who were worried above all about keeping Germany down after World War II. In signing on, the 12 original members who joined in 1949 agreed to uphold peace and international law among themselves. And importantly, they agreed to Article 5, which can obligate member states to come to one another’s defense should one of them be attacked in continental Europe or North America (or in territories north of the Tropic of Cancer). An additional 16 countries have joined since the alliance’s founding.

During the Cold War, though, keeping Russia out became priority one. It stayed a priority, to one degree or another, even after the breakup of the Soviet Union. In 2014, with Russia’s invasion of Ukraine raising concerns that a NATO state could be next, the alliance made its most formal statement about minimum defense spending “obligations” each member owed. Each country, the alliance stated, should try to meet the goal of spending 2 percent of its GDP on defense within a decade. It was those “obligations” Trump was referring to—but unlike the Article 5 collective-defense requirement, the spending target is not legally binding.

Trump’s comments throw the “keeping America in” function of NATO into question for the first time. I asked Michael Mandelbaum of Johns Hopkins University’s School of Advanced International Studies, who is an expert on NATO and American foreign policy, what it would mean if Trump put his ideas about the alliance into practice, and about what role the alliance has played historically. Mandelbaum is the author of Mission Failure: America and the World in the Post-Cold War Era. In addition to detailing how NATO has helped constrain European nations from fighting among themselves, Mandelbaum followed up after our conversation to note one more benefit of the alliance: “NATO has been an effective measure against nuclear proliferation.” Security guarantees may have helped prevent countries like Germany and Japan from seeking their own nuclear weapons (a legacy Trump has also questioned). Our conversation has been edited and condensed for clarity.

Nicholas Clairmont: If a NATO country were invaded [and invoked] Article 5, and the other member states didn’t come to its defense, what would happen?

Michael Mandelbaum: Well, they would be violating their treaty obligations. And so you would have to assume that the North Atlantic Treaty and NATO as a military organization would become null and void.

Clairmont: One of the positive effects of NATO that is sometimes touted is that NATO countries generally don't go to war with one another. Is that valid?

Mandelbaum: That has generally been true. You might make an exception for the Turkish invasion and occupation of the northern part of Cyprus.

NATO turned out to part of the solution to the problem that had bedeviled and in some ways devastated Europe for 75 years, between the beginning of the Franco-Prussian War and the end of World War II. And that is the German problem, which was how to fit Germany into Europe in a way that was acceptable both to Europe and to Germany. Dividing Germany, and enveloping its two parts in military alliances led by a stronger power, turned out to be a stable solution. So, it did serve that purpose. And it certainly helped to deter the Soviet Union. There’s a lot of debate about whether Stalin or Krushchev was ever really serious about invading. But it’s an unanswerable question even with the Russian documents, and we don't have all of them. And it’s particularly unanswerable, if I can use that ungrammatical construction, because we don’t know what Soviet attitudes would have been if there had been no NATO.

Clairmont: What do you think about Trump’s comments about NATO in general? Do you think making them was a good idea?

Mandelbaum: Well, they were certainly irresponsible. Although you have to qualify that, because to call them irresponsible might imply that Trump really had an understanding of what he was doing. And I don’t get the impression that he does.

I think his two defining features are his temperament, and his ignorance.

Clairmont: His claim is: It’s bad for the U.S. to go on sustaining NATO, because we pay a great deal more for our defense, by percent, than do a lot of other NATO members. And that’s the only reason the alliance is sustainable, and that we need to make a credible threat that America is willing to walk away and stop basically footing the bill for NATO, in order to get everyone else to pay up. One of the things I’m exploring is that he has not understood how much value NATO provides to the United States.

Mandelbaum: He looks at everything as a real estate deal—that we're not getting enough.

I would make two points. One is that, although the burden of the common defense is a bit lopsided—with the United States paying more than what American administrations have considered our fair share—it’s not as lopsided as Donald Trump seems to think. America’s allies really do make contributions. Especially in Asia. And, it also must be borne in mind that the United States has a global military. So, a lot of the American defense budget, and the budget that can be assigned to NATO or to Japan, is naval and air force. Which, presumably, the United States would want to have anyway. Maybe not to the same extent, but the Navy is a senior service. We’ve had one since the early 19th century. We're not going to give it up. So that's the first point.

The second point is: I do think that one consequence of what Trump has been saying, and what Obama said in the Jeffrey Goldberg interview [for The Atlantic cover story “The Obama Doctrine”], is that whoever is elected, there will be pressure to get the Europeans to pay more. If Mrs. Clinton is elected, she will feel that pressure, because it’s been placed on the national agenda as an issue.

Clairmont: Do you see a connection at all between Trump’s equivocation about honoring NATO Article 5, and Obama’s distinction between core and non-core interests, and [his discussion of] “free riders,” in “The Obama Doctrine”?

Mandelbaum: Well, they're connected by inference. But if you have signed a treaty to protect a country such as Estonia, Latvia, or Lithuania, that would seem to make it a core interest.

Clairmont: Russia has made military incursions in Chechnya, Georgia, Moldova, and Ukraine, all non-NATO countries. And one gets the sense that [Russian President Vladimir] Putin has designs on Estonia [as well as the other Baltic states Latvia and Lithuania], which are NATO countries. But he hasn’t done anything in those countries. Is this because NATO, so far, works?

Mandelbaum: I think the fact that Ukraine and Georgia were not in NATO certainly made them attractive targets. And now the Baltic states are in question. They’re not defensible, at least not with the force the United States and NATO have there. So they are in some sense the equivalent to the Cold War status of West Berlin. But Putin has lots of ways to harass the Baltics: cyberattacks, stirring up ethnic Russians. So, he could make a lot of trouble for Estonia, Latvia, Lithuania, without having Russian troops cross the border between them and Russia.

When NATO expansion was proposed it was presented by the Clinton administration as being a way to unite Europe. And those of us who were opposed 20 years ago said: “To the contrary, this is going to create a line of division in Europe.” And so it did. It would have been a line of division if only Russia had been excluded. But for various reasons Georgia and Ukraine were also excluded, and now they are in no-man’s land.

Clairmont: Walter Russell Mead, the foreign-policy writer and my former boss, sometimes says that if you put up signs over one half of a lake that say “no fishing,” people are going to make an assumption about the other half of that lake.

Mandelbaum: There is something to that.

I think that although NATO expansion was a terrible mistake—and a very costly one, in that Russia might well have a different foreign policy than it does if not for NATO expansion and all that followed—precisely because of what Russia has become, there is a need for NATO. Europe is important to the United States. But it’s true that the Europeans pay less than what every American president since Eisenhower regarded as their fair share—President Obama called the Europeans free riders, and to some degree indeed they are. They have been for over 60 years, dating back to 1952 and the Lisbon Agreement [on NATO Force Levels]. The idea was that NATO should have many more ground troops than it had, and they would come from the Europeans. But the Europeans never stumped up.

Clairmont: Can you tell me more about the Lisbon Agreement? The discussion of force levels did not begin until after the treaty was inked in ’49?

Mandelbaum: No, it was a few years afterwards. And there was another, later point at which the Kennedy administration, because of changes in the nuclear balance, adopted a policy of “flexible response,” which meant that there needed to be more NATO ground troops. And the Europeans agreed in principle, but never supplied them. I wrote about this in the first book that I ever published, called The Nuclear Question.

Clairmont: So, is the requirement to spend 2 percent as binding as the Article 5 collective self-defense requirement? Is it legally required as a term of membership?

Mandelbaum: No, it is not in the treaty.

Clairmont: Do you have any closing points?

Mandelbaum: The Europeans have been not quite been free riders, but they pulled less than their weight. And the case that we are paying an inordinate amount for collective defense is sort of true in the Pacific with Japan. Although, the United States does get economic benefits. That is, the Japanese pay a lot of the cost of the bases, and if we wanted to base American troops in the United States rather than overseas, it would be expensive. So NATO is not exactly a paying proposition, and it’s not intended to be a paying proposition.

But simply abandoning NATO would be costly, just in economic terms. And it would be very costly in geopolitical terms.

Clairmont: Is NATO worthwhile? Is the world a better, more peaceful place for America's being in NATO and being willing to honor Article 5?

Mandelbaum: Yes, it is.

Christopher I. Haugh contributed reporting.