Of all the warnings in President Trump's arsenal, quitting the NATO military alliance is among those he's wielded the most. Now he's doing it again.

Asked by Britain's Telegraph newspaper if he is reconsidering US membership of NATO, he said: Oh yes… I would say [it's] beyond reconsideration – fuming again that his partners weren't joining America's military operations, alongside Israel, against Iran.

I just think it should be automatic, he emphasized in his remarks to the paper.

Trump's invective underlines again his misunderstanding of how this 32-member alliance works. NATO's Article 5 does commit it to collective defense. An attack against one member is deemed to be an attack against all, but invoking this principle requires a consensus. The 1949 treaty only referred to crises in Europe and North America.

One ally after another has held back from joining a war they weren't consulted about, given they still don't understand its goals in the face of mixed messaging from the Trump administration. Article 5 has only been triggered once, in the wake of the September 11th attacks on the US in 2001.

After Russia's audacious full-scale invasion of Ukraine in February 2022, the then-US President Joe Biden took a leading role in shaping the response of individual Western governments because he believed President Putin's actions threatened them all. NATO, as an alliance, provided assistance but avoided the dangerous prospect of becoming directly involved as a party to this conflict.

Even before Trump entered the White House in 2017, he repeatedly dismissed NATO as a paper tiger, described it as obsolete, and said that it was costing a fortune for the US.

This year, he's mocked the alliance, saying Russia would have occupied all of Ukraine if the US had not been NATO's enforcer. Trump almost walked out in early 2019 during his first term in office. We saw clear signs that Trump was preparing to act on his threat, wrote former NATO Secretary General Jens Stoltenberg in his memoirs.

Stoltenberg recounted how he went on Fox News and credited Trump with pressuring NATO allies to increase their military spending, acknowledging social media praises but not making a speech reportedly drafted for a US pullout.

At the center of Trump's concerns was the 2014 agreement that countries should spend 2% of their GDP on defense; at the time, it was only described as a guideline. Military spending has ramped up significantly by almost all NATO members, partly in response to Trump's threats, partly due to Russia's growing menace.

This new crisis will again strengthen the resolve of European countries and Canada to bolster their own defenses and depend on themselves for their security. But there exists the cold hard fact that the might of the US military matters immensely.

The US's military budget now makes up some 62% of NATO's total defense spending, with the Pentagon possessing unmatched assets and intelligence capabilities.

This time, Secretary of State Marco Rubio has also chimed in, stating, I think there's no doubt, unfortunately, after this conflict is concluded, we are going to have to re-examine that relationship.

Although he lamented that not using US bases to defend America's interests made NATO a one-way street, the US Congress voted at the end of 2023 prohibiting the president from unilaterally withdrawing from NATO without a two-thirds Senate majority approval or an act of Congress.

NATO leaders, including current Secretary General Mark Rutte, will need to work hard to convince Trump that staying aligned is beneficial for both America and the alliance, as they face myriad threats at both home and abroad.