Article 5 is the principle that an attack on one member of NATO is an attack on all members. It's been a cornerstone for the 29-member alliance since it was founded in 1949 as a counterweight to the Soviet Union.

Article 5 has only been invoked once: After the September 11, 2001, terror attacks on the US.

Article 5 aims to deter potential adversaries from attacking NATO members. During the Cold War, the main concern was the Soviet Union, but in recent years, Russia's aggressive actions in Eastern Europe have been the focus of attention. Ukraine and Georgia, the two countries Russia has invaded in the past decade, are not NATO members.

When Trump came into office, however, Eastern European countries like Poland were nervous about whether he would actually uphold the principle because of his campaign rhetoric.

In addition to calling NATO "obsolete," Trump has railed against NATO allies in Europe for not allocating 2% of their GDP toward defense.

"The countries we are defending must pay for the cost of this defense, and if not, the US must be prepared to let these countries defend themselves," Trump said in an April 2016 foreign policy speech. "We have no choice."

During his first visit to NATO headquarters as President, Trump notably omitted any commitment to Article 5 from his speech, alarming allies and critics, and instead scolded them for not meeting their financial obligations.

"Twenty-three of the 28 member nations are still not paying what they should be paying and what they are supposed to be paying for their defense. This is not fair to the people and taxpayers of the United States," he said. The remarks were delivered shortly before Montenegro joined the alliance as its 29th member.

But Trump changed course last month during a joint news conference with the Romanian president, saying, "I am committing the United States to Article 5."

"And certainly we are there to protect," Trump added, saying this is why the US is "paying the kind of money necessary to have that force."

He reiterated that stance while in Poland, one of the Eastern European countries most concerned with Russia's aggressiveness.

"To those who would criticize our tough stance, I would point out not merely with words but with its actions that we stand firmly behind Article 5, the mutual defense commitment," Trump said. "Words are easy but actions are what matter and for its own protection, and you know this, everybody has to know this, Europe must do more."

In 2014, NATO countries agreed to try to spend 2% of their GDP on defense, although most countries fail to hit that benchmark, which is not binding.

NATO members are increasing their defense spending collectively, however, and more countries are expected to hit the 2% benchmark in the coming years.

NATO's Article 5 principle stretches beyond attacks on the homeland. The alliance has also taken collective defense measures on several occasions, including deploying Patriot missiles in 2012 on the Syrian-Turkish border and bolstering its forces in Estonia, Latvia, Lithuania and Poland following Russia's annexation of Crimea in 2014.

NATO allies also have joined the US to fight the wars in Afghanistan, Iraq and Syria.

To critics, Trump's rhetoric about countries failing to meet the 2% benchmark ignores the contributions they have made providing troops to the fights against al Qaeda and ISIS, and he risks alienating allies at a time the US could be seeking additional troops from NATO countries in Afghanistan.

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What is Article 5? (And why it matters) - CNN International