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 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 on 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 S. Truman has when speaking outside 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 gross domestic product.

"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 was the US's 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 on 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 he was "not here to lecture" them about human rights.

Richard Haass, a former US diplomat who has been the 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 Haass called "unseemly and counterproductive."

Ivo Daalder, the US's ambassador to NATO from May 2009 to 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. "At the core of NATO is the unconditional commitment to collective defense."

'Putin will be thrilled'

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

NATO was founded in 1949 as Europe's answer to the Soviet Union, and the 28-member alliance continues to serve largely as a counterweight to Russia's ambitions in eastern Europe. Several post-Soviet states, including Lithuania, Latvia, and Estonia, are now NATO members. Montenegro will become a member in June.

"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." (Trump said later that he called NATO obsolete "without knowing much about NATO.")

Still, some officials worried that Trump could one day strike a bilateral deal with Moscow that would affect 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 became 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.

View photos FILE PHOTO: Russian President Vladimir Putin and German Chancellor Angela Merkel shake hands during a joint news conference following their talks at the Bocharov Ruchei state residence in Sochi, Russia, May 2, 2017. REUTERS/Alexander Zemlianichenko/File Photo More

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