Collective self-defense is enshrined at NATO’s very core: Article 5 of the North Atlantic Treaty directly states that “an armed attack against one or more of them in Europe or North America shall be considered an attack against them all” and that force can be used in response.

It’s also a relatively untested commitment — and an increasingly fraught one. Since NATO’s founding in 1949, it has been invoked only once: On Sept. 12, 2001, after the terrorist attacks in the United States the day before.

Seventy years after the founding of NATO, it’s not clear that all allies are on the same side about what collective self-defense would actually mean. President Trump has emerged as a powerful critic of the alliance, while Turkey’s intervention in northeastern Syria raised concerns about just how far collective security would go.

In a recent interview with the Economist, French President Emmanuel Macron wondered whether NATO’s collective security could result in his country fighting a war against Syria. “What will Article 5 mean tomorrow?” Macron wondered aloud.

It’s not clear whether members of the public are clear on that either.

Earlier this year, British polling firm YouGov quizzed people in four major NATO powers — Britain, France, Germany and the United States — about what foreign nations they would be willing to defend, including NATO allies and some non-NATO allies such as Sweden and Ukraine.

The results suggested that being a NATO ally did not ensure public support.

Only minorities of the populations in France and Germany said that they would not defend Romania, a NATO ally since 2004 and a European Union member, or even Turkey, a NATO ally since 1952 (Britons were also divided on support for Turkey).

However, in all countries polled, majorities said they would support Finland and Sweden, which are not part of NATO. Majorities in Britain and the United States also said they would come to the aid of Ukraine, a country that was blocked from joining NATO in 2008 — confusingly, a decision pushed by the United States at the time.

And although Germany and the United States have a decades-long alliance, only a minority of Germans said they would come to the defense of Americans if the United States were attacked

YouGov’s poll was conducted in April, but shared again Monday in anticipation of a NATO meeting in London this week during which Trump and other world leaders will celebrate the anniversary of the alliance’s founding.

The results prompted debate among European security experts, with many wondering how to parse the information, whether the question itself was phrased in a confusing or misleading way, or even whether the polling data could be considered accurate at all (comparative international polling is an expensive and difficult business, with lots of opportunities for error and misinterpretation).

Some other surveys have found similar levels of national disagreement about how Article 5 would work in practice, although the takeaways are often complicated. In 2017, the Pew Research Center asked people in a variety of NATO member states whether they would respond with military force if a NATO ally got into a serious conflict with Russia.

Only 40 percent of Germans favored using military force in this situation, compared with 45 percent of Britons, 53 percent of the French and 62 percent of Americans.

Germany may be notable for its mixed feelings about the alliance. A poll conducted this year by the German company Körber-Stiftung found that many in the country were skeptical about key parts of its NATO relationship, with only 39 percent saying that Germany should have a closer relationship to the United States rather than Russia.

Whether these polling numbers convey a shifting view of NATO collective security among the public may not matter — national security decisions are made not by referendum.

Trump, who was a fierce critic of NATO on the campaign trail and still complains regularly about European military spending, eventually committed to Article 5 in June 2017 during a White House news conference with Romanian President Klaus Iohannis.

But only a year later, Trump suggested that Montenegro, a small nation that joined NATO last year, could lead to a global war. “They are very aggressive people. They may get aggressive, and congratulations, you’re in World War III. But that’s the way it was set up,” Trump said during an interview with Fox News in July 2018

“Don’t forget, I just got here a little more than a year and a half ago,” Trump added.

During a recent interview, NATO Secretary General Jens Stoltenberg said that Article 5 was the “cornerstone of NATO” and that the fact that no European nation had been attacked since NATO started was “perhaps the greatest success” of the organization.

But if NATO’s collective security serves best as a tool for deterrence, it relies on the perception that it would actually work in practice. The risk now is that if it ever had to be used in practice, world leaders might find it is a relic of the past that no longer functions.