Cracks are beginning to appear in a decades-long effort by America to be NATO’s go-to source for fighter aircraft.

Some European partners are irked by President Trump’s increasingly expansionist rhetoric and approach to the Russia-Ukraine war, while others question what the president’s criticisms of NATO mean for America’s future in the alliance.   

Things have gotten so bad that this week the Pentagon had to squash rumors that a “kill switch” lurked in the planes’ hardware and European press outlets breathlessly warned of secret algorithm-driven “black boxes” attached to the aircraft that are beholden only to their makers in America.

One Danish politician warned that he could envision Washington and its friends in the arms industry “de-activating” the planes in order to help capture Greenland. The unfounded claim was quickly squashed by the Pentagon

“You’re not just buying an airplane, you’re buying a relationship with the United States,” Winslow Wheeler said, who spent 30 years on Capitol Hill working on national security issues for both Republican and Democrat senators. “People in the past have not just welcomed but craved that kind of relationship.”

The F-35 program reflects an effort by America to produce a best-in-class aircraft that can be flown by all members of the transatlantic alliance. The fighter jets, produced by Lockhead Martin, are bolstered by a joint communication platform and technology, making it advantageous for sharing information among the allies. The program has served as a core component of NATO for years and 19 of America’s allies currently operate them. 

As NATO members begin to replace their aging fleets, some now are considering pumping the brakes on their existing F-35 contracts.

This week, a member of Denmark’s Conservative People’s Party, Rasmus Jarlov, voiced his regret over approving his country’s purchase of the F-35 Joint Strike Fighter, suggesting that even in the absence of a “kill switch,” America could “disable the planes” simply by “stopping the supply of spare parts.” 

The Danish parliamentarian upped the ante by claiming that he could “easily imagine a situation” in which America threatens to “deactivate” Denmark’s American-supplied weapons as a ploy to acquire Greenland.

Mr. Trump’s musings about acquiring the Danish territory “one way or another” have not gone down well at Copenhagen. The 47th president has set his eyes on Greenland, which he says is vital “for national security,” although the Arctic island is an autonomous territory within Denmark. 

Mr. Jarlov continued: “Therefore, buying American weapons is a security risk that we can not run. We will make enormous investments in air defence, fighter jets, artillery and other weapons in the coming years, and we must avoid American weapons if at all possible.” Mr. Jarlov signed off by encouraging Denmark’s “friends and allies to do the same.” 

Lockheed Martin, for its part, stated this week that it is committed to delivering “all system infrastructure and data required for all F-35 customers to sustain the aircraft,” the company says. 

The wariness over F-35s is part of a greater movement to reduce Europe’s dependence on America, particularly concerning military equipment. Concerns have mounted as Mr. Trump has voiced increasingly critical comments about NATO, suggesting that European countries should take on greater costs of maintaining the alliance.

In recent years, more than half of Europe’s member countries have placed about two-thirds of their equipment orders with American companies. Though such a trend may be nearing its end, with the European Union on Wednesday publishing its “Readiness 2030” security strategy, which expands the EU’s defense spending but directs member countries to purchase weapons and equipment from European suppliers instead of those abroad when possible. 

“The security architecture that we relied on can no longer be taken for granted,” the president of the European Commission, Ursula von der Leyen, said. “Europe is ready to step up. We must invest in defence, strengthen our capabilities, and take a proactive approach to security.” She later urged: “We must buy more European.” 

In Canada, a founding member of NATO, tension over tariffs and Mr. Trump’s comments about making it the 51st American state, have prompted the new prime minister, Mark Carney, to ask his defense minister, Bill Blair, to review potential alternatives to F-35s. A spokesman confirmed that Mr. Blair is reconsidering the fighter jets “given the changing environment.”