The reliability of the North Atlantic Treaty Organization will be a question Thursday at a meeting in Brussels of defense ministers of the pact and again over the weekend at the annual Munich Security Conference. President Trump’s remarks on NATO have sharpened arguments that Europe must invest in its own deterrent against  Russia.

As the leaders of three European heavyweights — France, Germany, and Poland — met Monday in Paris to discuss defense, Poland’s prime minister, Donald Tusk, said: “It is probably here in Paris that the words from ‘The Three Musketeers’ by Alexandre Dumas resonate most clearly: ‘All for one, and one for all.’”

That used to be NATO’s line. Under Article 5 of the 31-nation mutual defense treaty, an attack on one member is to trigger military support from all the others. On Saturday, though, Mr. Trump ratcheted up his long running disdain for the 75-year-old pact, inviting Russia to attack any member that is not spending its fair share on defense.

The shock caused by Mr. Trump’s remarks has sharpened arguments across the pond that Europe must invest in its own deterrent to keep Russia at bay. The reliability of NATO will be a major question at Thursday’s meeting of NATO defense ministers at Brussels, and over the weekend at the annual Munich Security Conference.

Mr. Trump’s erstwhile national security adviser, John Bolton, predicts in a new edition of his 2020 memoir that if elected to a second term, Mr. Trump would try to pull out of the North Atlantic Treaty. From Germany, Bundestag foreign affairs committee member Norbert Röttgen writes  on Facebook: “Everyone should watch this video of Trump, and then understand that Europe may soon have no choice but to defend itself.”

At Paris, one goal of Monday’s meeting was to recreate the “Weimar Triangle,” a working alliance of three countries which have more than four times Russia’s gross domestic product. Addressing Mr. Tusk, President Macron said: “It’s a joy to have you back and have, through your government, partners whom we can trust, are pro-European, and clear on European security.”

At Brussels, the EU’s internal market commissioner, Thierry Breton, told France’s LCI television Sunday that Europe has to boost its military spending, adding: “We cannot flip a coin about our security every four years depending on this or that election, namely the U.S. presidential election.” The Financial Times editorialized Monday: “Europeans have to start thinking the unthinkable: how to plan for war without America.”

Alarmed by Russia’s war on Ukraine, Russia’s other western neighbors are already taking concrete actions. Until 1918, Finland, Estonia, Latvia, and Lithuania were ruled by Moscow. Now, all are NATO members. Finland, which joined NATO last year, just extended through April 14 the closure of all nine crossings on its 830-mile long land border with Russia.

Next year, the three Baltic nations are to start building the “Baltic Defense Line” — a string of 4,500 concrete bunkers along their 535-mile long border with Russia. Designed to slow an invasion force, the 400 square foot bunkers would hold 10 soldiers and be capable of taking a hit from an artillery shell. Poland, another former Russian possession, also is fortifying its border with Russia and with Belarus, a Russian satellite.

Europe’s bigger countries cite the threat of Russia in order to prepare their citizens for more defense spending. “It cannot be ruled out that within a three- to five-year period, Russia will test Article 5 and NATO’s solidarity,” Denmark’s defense minister, Troels Lund Poulsen, told the newspaper Jyllands-Posten last week. “That was not NATO’s assessment in 2023. This is new information that is coming to the fore now.”

Over the last month, similar public warnings have come from Germany’s defense minister, Boris Pistorius; Estonia’s prime minister, Kaja Kallas; and the head of Great Britain’s Army, General Patrick Sanders. The Swedish prime minister, Ulf Kristersson, has warned his citizens to prepare to defend themselves “with weapons in hand and our lives on the line.”

Romania’s newly appointed army chief, General Vlad Gheorghiță, told Radio Free Europe last week: “The Russian Federation will not stop here. If [Putin] wins in Ukraine, the main target will be the Republic of Moldova. We will witness tensions in the western Balkans. I am more than convinced that President Putin’s policy will escalate in the immediate future.”

Mr. Trump’s badgering as president, followed by Russia’s invasion of Ukraine on President Biden’s watch are credited with raising the number of NATO countries meeting a target of spending at least two percent of GDP on defense. This year, Germany is expected to join 11 nations that have met the target, first set a decade ago.

Public support can be difficult to maintain. According to the latest Munich Security Index, Germans’ worry list is topped by immigration and radical Islam. Russia has fallen to seventh place. Last year, America spent 3.5 percent of its national output on defense, and Poland spent four percent. Now, with new American aid for Ukraine held up in Congress, Europe has displaced the United States as the top supplier of aid to Ukraine. 

Europeans believe it is best to stop Russia’s military on Europe’s far eastern edge. According to Ukraine’s defense ministry, the nearly two-year long war has cost Russia 6,416 tanks, 11,977 armored personnel carriers and 395,990 soldiers killed or severely wounded. Some European nations are giving Ukraine almost all their entire fighter jets. Over the next two years, Norway is giving Ukraine 10 F-16s, Belgium is giving over 50 F-16s, and Netherlands and Denmark a total of 61 F-16s.

“Not only the United States, but all European countries must do even more to support Ukraine,” Chancellor Scholz of Germany said Monday on a visit to the future site of a $300 million Rheinmetall artillery shell factory. “The pledges made so far are not enough. Germany’s power alone is not enough.”

Last month, Mr. Scholz announced plans to double this year military aid to Ukraine. At the same time, Prime Minister Sunak visited Kyiv to sign a landmark bilateral security pact and to confirm a record annual package of over $3 billion. The foreign secretary, David Cameron, said: “I have no doubt we can make sure Putin loses — and it’s essential that he does lose.”

President Macron was to visit Kyiv Monday. He decided instead to meet the German and Polish leaders at Paris. Later, he is to sign a similar security pact with Ukraine and to deliver record military aid, including 40 long range Scalp cruise missiles. Writing before Mr. Trump’s criticism of NATO, Atlantic Council expert Diane Francis wrote: “There is an emerging consensus among European leaders that it is unwise to rely on the United States to defend them against Russia.”

“They appear to recognize that Europe must be able to meet its own security needs, and are mobilizing to provide Ukraine with the backing necessary to prevent a Russian victory that would have disastrous consequences for the entire continent,” Ms. Francis writes in an essay titled: “Europe steps up support for Ukraine in fight against Putin’s Russia.”