Trump again questions NATO's collective defense guarantee ahead of summit

THE HAGUE, Netherlands — President Donald Trump on Wednesday once again raised questions about America's commitment to defend its allies should they come under attack as he prepared to join a NATO summit in the Netherlands.

Just as he did during his first term in office, Trump suggested that his backing would depend on whether U.S. allies are spending enough on defense. He's demanded that European allies and Canada dedicate 5% of GDP to their security.

On the eve of the meeting in The Hague, Trump told reporters aboard Air Force One that his commitment to Article 5 of NATO's founding treaty – the organization's collective security guarantee – ''depends on your definition.''

''There's numerous definitions of Article 5. You know that, right?'' Trump said. ''But I'm committed to being their friends.'' He signaled that he would give a more precise definition of what Article 5 means to him once he is at the summit.

As a candidate in 2016, Trump suggested that he as president would not necessarily heed the alliance's mutual defense guarantee. In March this year, he expressed uncertainty that NATO would come to the United States' defense if needed.

What Article 5 says

Article 5 is the foundation stone on which the 32-member North Atlantic Treaty Organization is built. It states that an armed attack against one or more of the members shall be considered an attack against all members.

It also states that if such an armed attack occurs, each member would take, individually and in concert with others, ''such action as it deems necessary, including the use of armed force, to restore and maintain the security of the North Atlantic area.''

That security guarantee is the reason previously neutral Finland and Sweden sought to join NATO after Russia launched its full-scale invasion of Ukraine in 2022 and why Ukraine itself and other countries in Europe also want in.

When it has been invoked

Article 5 was only invoked once, in the wake of the September 11, 2001, terror attacks on the United States, paving the way for NATO's biggest ever operation in Afghanistan.

But NATO allies have also taken collective defense measures, including joining the U.S. to fight the Islamic State group in Syria, Iraq and Afghanistan, as well as help keep the peace in the Balkans.

The Three Musketeers-like pledge of all for one, one for all, is at the heart of NATO's deterrent effect. To question it too loudly might invite an adversary to test it. European officials have said that Russia is planning to do just that.

The impact of Article 5 on Ukraine

NATO's credibility hinges on Article 5 and its commitment to offer membership to any European country that can contribute to security in Europe and North America.

But Ukraine, currently in the middle of war with Russia, might oblige all 32 member countries to spring to its defense militarily, potentially igniting a wider war with a nuclear-armed country. Trump is vetoing its membership for the foreseeable future.

Article 5 becomes problematic when the territory of a member is unclear. For instance, Russian forces entered Georgia in August 2008, a few months after NATO leaders first promised the country it would join, along with Ukraine.

Georgia's NATO application is still pending but seems unlikely for many years. Russia continues to occupy large swaths of Ukraine and other parts are contested, meaning that its borders cannot be easily defined.