President Donald Trump, flanked by members of law enforcement, speaks before signing bills in the Diplomatic Reception Room at the White House, Friday, June 2, 2017, in Washington. AP Photo/Alex Brandon President Donald Trump's own national security team was reportedly blindsided after he did he did not explicitly endorse Article 5 of the North Atlantic Treaty Organization's founding document during his summit with NATO allies in Brussels last month.

And it looks like all he did was omit one simple sentence.

"We face many threats, but I stand here before you with a clear message: the U.S. commitment to the NATO alliance and to Article 5 is unwavering," Trump was supposed to say, according to a speech prepared by his national security team that was shared with Politico on Tuesday.

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.

Asked about the omission on Tuesday, White House press secretary Sean Spicer told reporters that it was "a bit of a silly discussion," because the NATO summit itself had been organized to celebrate Article 5.

"I don't know about the contents of the speech but frankly it's a bit of a silly discussion," Spicer said, adding that Trump's presence "pretty much speaks for itself in terms of our commitment to NATO and all 13 articles that make up that treaty."

But five sources told Politico on Monday that US Secretary of Defense Jim Mattis, national security adviser H.R. McMaster, and Secretary of State Rex Tillerson worked with Trump on the speech for weeks and pushed hard for the Article 5 language to be included after viewing later drafts in which it had been taken out.

Trump said in his speech that the US would "never forsake the friends that stood by our side" in the aftermath of the 9/11 attacks. 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 gross domestic product.

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."

Other current and former NATO officials were left reeling by the omission, with many speculating that Russian President Vladimir Putin would be overjoyed by it.

"Putin will be thrilled at Trump's refusal to endorse Article 5," 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, said at the time. "Unimaginable under any other president."