NATO "cannot be an 'a la carte' military alliance, it cannot be a military alliance that works depending on the humor of the president of the US," Josep Borrell, the foreign policy chief of the European Union (EU), told reporters in Brussels in response to Donald Trump's recent comments about NATO. 

On Saturday, at a campaign rally in South Carolina, Trump had told the crowd that, as president, he had warned NATO allies that he "would encourage" Russia "to do whatever the hell they want" to countries that did not "pay [their] bills." His comments sent a chill across Europe, alarming NATO's European members, already agitated about the prospect of a second Donald Trump presidency. 

"Any suggestion that allies will not defend each other undermines all of our security, including that of the US, and puts American and European soldiers at increased risk," NATO Secretary-General Jens Stoltenberg responded in a statement on Sunday.

NATO grapples with Trump's threats 

As president, Trump had threatened to withdraw from NATO many times. He warned he would make Europeans pay for America's protection and repeatedly threw into doubt the US commitment to the core of the alliance — a promise enshrined in Article Five of the North Atlantic Treaty providing that "an armed attack against one or more of them in Europe or North America shall be considered an attack against them all." 

The fact that he is doing this again — attacking "the soul of the alliance," as some at NATO view it — this time on the campaign trail, has been described by diplomats as "worrying." After all, many allies fear a potential second presidency might feature an unhinged and much bolder Trump than during his first tenure.

"The last time that Trump was in office, it was the biggest upheaval in the Europe-United States relations since the beginning of the EU," Alison Woodward, a senior associate fellow with the Institute for European Studies in Brussels, told DW. 

"It was really a very dramatic change," she added. "And so, I think the leaders now are bracing themselves for what could possibly occur if Donald Trump does get reelected." During Trump's first presidency, the US had imposed punitive tariffs on trade with EU members, noticeably chilling trans-Atlantic relations.

A critical time for NATO

Trump's recent remarks come at a critical time for the alliance, with some allies openly warning about a potential escalation of Russia's war in Ukraine, while a new US aid package for Kyiv is stalled in Congress and Europe is struggling to ramp up its weapons production. 

Trump's comments "increase the probability that NATO will be tested by Russia, especially if Donald Trump would win the elections, but perhaps not only then," said Michal Baranowski, managing director of the German Marshall Fund East, a US think tank.

"The comments made Europe less secure," he told DW, adding that Trump "created a question in the minds of many leaders, including on NATO's eastern flank, whether the US will stand with all the allies in case of an attack on one of them." 

These worries are echoed by diplomats in Brussels, who privately say that Trump's comments have already inflicted damage to the alliance. The biggest problem seems to be that his claims are so difficult to rebuke. Trump lashing out at NATO allies for "not paying their bill" is a misleading statement because technically, there is no bill to pay. 

Trump's comments a wake-up call?

Trump's comment was referring to the fact that a significant number of NATO member states are still spending less than 2 percent of their GDP — a goal agreed at a NATO summit in Wales in 2014. 

Germany is expected to meet the target this year for the very first time since the end of the Cold War, mostly thanks to a special fund of €100 billion ($107 billion) created in response to Russia's full-scale invasion of Ukraine in 2022. But further funding is far from guaranteed. 

That is why diplomats and experts in Brussels agree that Trump does have a point when it comes to the urgent need for Europeans to invest more in their collective defense. "I think what the presidential candidate in America said is also something to wake up some of the allies who haven't done that much," Estonia's Prime Minister Kaja Kallas told journalists during a visit to Brussels. 

Europe's contingency plans

Governments across the continent seem to understand European allies need do more for their own defense — regardless of who will be the next US president. Those efforts are at the heart of the contingency plans European allies are working on behind the scenes, focusing on enhanced military capabilities and a more unified strategic approach. 

But there's a long way to go, says American government analyst Bart Kerremans, a political science professor at Belgium's Leuven Institute for International and European Studies. "The prospect of a second Trump presidency provides a strong impetus on enhanced European cooperation and integration with respect to defense and security," Kerremans said, pointing out it is not only about Donald Trump.

"If you want to reduce the likelihood that under Trump, the US would withdraw itself from NATO, you have to do something about burden sharing. And apart from Trump, if Europe wants to play an important role in the world, it will have to invest more in defense as the world is getting less and less secure."

Edited by: Maren Sass