'Putin will be thrilled': Former NATO ambassadors say Trump just dealt 'a major blow' to the alliance

President Donald Trump drew backlash Thursday after he did not explicitly endorse Article 5 of the North Atlantic Treaty Organization's founding document during his summit with NATO allies in Brussels.

The article, known as the collective defense clause, stipulates that that an attack on any member is an attack on all. It was invoked for the first time in response to the September 11, 2001, terrorist attacks - a point raised by Belgian Prime Minister Charles Michel and NATO Secretary General Jens Stoltenberg in their respective remarks Thursday.

Trump said in his speech that the US would "never forsake the friends that stood by our side" in the aftermath of 9/11. But he did not explicitly endorse Article 5, as every US president since Harry Truman has done when speaking outside of NATO headquarters.

Instead, Trump used the speech largely to lecture representatives from nearly two-dozen member countries for not meeting their "financial obligations" to increase defense spending to 2% of their GDP.

"If NATO countries made their full and complete contributions, then NATO would be even stronger than it is today, especially from the threat of terrorism," Trump said.

Nicholas Burns, who served as the US Ambassador to NATO under President George W. Bush, said it was "a major mistake" for Trump to not "reaffirm publicly and explicitly" the US's Article 5 commitment to NATO.

"I was the US Ambassador to NATO on 9/11 and remain grateful for the unstinting support given to America by our European allies and Canada," Burns said Thursday. "Trump is not acting like the leader of the West that all US presidents before him have been dating back to Truman."

Trump's speech at the NATO summit came on the heels of his trip to the Middle East, where he told Arab leaders that he was "not here to lecture" them about human rights. Richard Haas, a former US diplomat who has been president of the Council on Foreign Relations since 2003, said on Twitter that Trump's overly-solicitous treatment" of Saudi Arabia stood in contrast to his "public lecturing of NATO allies," which he called "unseemly and counterproductive."

Ivo Daalder, another former US ambassador to NATO who served between May 2009 and July 2013, said Trump's reluctance to commit to the guiding principle was "a major blow to the alliance."

"After calling NATO 'obsolete,' Trump needed to say what every predecessor since Truman has said: The US is committed to Article 5," Daalder said on Twitter. "He didn't. This is a major blow to the Alliance. At the core of NATO is the unconditional commitment to collective defense."

'Putin will be thrilled'

NATO officials reportedly hoped that Trump would acknowledge that the organization's biggest challenge now is not fighting terrorism but countering Russian aggression in eastern Europe, according to Politico.

NATO has 28 member countries. The alliance serves, among other things, as a counterweight to Russia's ambitions in Eastern Europe - it was founded in 1949 as Europe's answer to the Soviet Union. Several post-Soviet states, including Lithuania, Latvia, and Estonia, are now NATO members. Montenegro joined recently.

"Putin will be thrilled at Trump's refusal to endorse Article 5," said Tom Wright, the director of the Center on the United States and Europe and a fellow in the Project on International Order and Strategy at the Brookings Institution. "Unimaginable under any other president."

After Trump called NATO "obsolete" in a January interview, Russian President Vladimir Putin's spokesman said Moscow "shares Trump's opinion that NATO is a remnant of the past." Some officials worried afterward that Trump could one day strike a bilateral deal with Moscow that would impact NATO's interests, Politico reported.

Putin has repeatedly characterized the US-led organization as an "aggressive" force whose aim is to isolate Russia from Europe - rhetoric that grew more heated earlier this year amid NATO's military exercises in the Baltic Sea. Russia responded to those drills by transferring nuclear-capable missiles to Kaliningrad, which borders Poland and Lithuania.

Daalder, the former US NATO ambassador, noted that 23 NATO member countries have increased their defense spending since last year and that eight countries "will spend 2% on their military next year."

"All allies committed in 2014 to spend at least 2% on defense by 2024. They did so because of Russia's invasion of Ukraine," he said.

Stoltenberg told reporters Thursday that he and Trump are "on the same line" when it comes to the conflict in Ukraine, where Russia intervened in 2014 to support the pro-Russia separatist movement after annexing the peninsula of Crimea. But Stoltenberg said he hadn't yet found a "common position" on Russia with Trump.

"Trump could have thanked allies for increasing their spending and urged them to accelerate the timetable by a few years," Daalder continued. "Certainly, the threat from Russia warrants increased spending. NATO countries see Russia as the greatest threat to their security."

German Chancellor Angela Merkel reportedly brought a map of the former Soviet Union to her meeting with Trump at the White House in March to show him what Putin was nostalgic for - a vast empire that extended well past Russia's current western border.

"What is needed is a clearly formulated American policy on Russia," Anders Fogh Rasmussen, who served as NATO's secretary general until 2014, told Politico. "The reason why people are preoccupied by all the investigations [in Washington] is that there is no clear Russia policy."

But the topic was purposefully kept off the agenda for the alliance's first major meeting with Trump on Thursday, a NATO spokesperson confirmed to BuzzFeed earlier this week. The leaders were apparently hoping to curry favor with Trump so that he would "explicitly" state his support for Article 5 before being asked about Russia, a European official told the publication.

Even so, "Russia is of course the elephant in the room" in the Europeans' discussions with Trump, an official told Politico.

White House press secretary Sean Spicer told reporters later that Trump did not need to explicitly endorse the collective defense clause in his speech because "the entire ceremony was called an Article 5 dedication."

"We're not playing cutesy with this," Spicer said. "He's fully committed. If you are standing at a ceremony talking about the invocation of Article 5 after 9/11 and talking about that, that is a pretty clear indication of the support that exists for it."

Another senior administration official said Trump's push for NATO members to spend more on their own defense was "obviously making life more difficult for Russia" and "creating a stronger and more vibrant Europe."

"The more NATO countries spend, the worse it is for Russia," the official said. "What Trump is doing, really, is increasing NATO's ability to deter any kind of aggression on its borders, including from Russia."