President Donald Trump holding a rally at the Kentucky Exposition Center in Louisville, Kentucky. Thomson Reuters President Donald Trump drew backlash last month 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. And it appears his own national security team was blindsided by it.

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.

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.

Lower-level members of Trump's national security team, meanwhile, were blindsided, according to the report.

"There was a fully coordinated other speech everybody else had worked on" — and it wasn't the one Trump gave, a White House official told Politico. Another national security source told Politico that the team "didn't know it had been removed."

"It was only upon delivery," the person said.

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.

It is unclear whether Trump deleted the line of his own volition or whether advisers such as Steve Bannon or Stephen Miller — each of whom views NATO with deep skepticism — pressured him to take it out.

But it is indicative of how much influence the antiglobalist faction of the White House — led by Bannon, the former chairman of the far-right website Breitbart News — has over Trump, who has shown a willingness to discard the advice of his more politically experienced advisers in favor of guidance from a small group of unseasoned but loyal confidants.

National Security Council spokesman Michael Anton did not dispute the account, telling Politico that Trump's presence at the summit was an inherent show of support for Article 5.

"His continued effort to secure greater defense commitments from other nations is making our alliance stronger," Anton said.

Anton appears to have been echoing the White House press secretary, Sean Spicer, who told reporters last month 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."

'Putin will be thrilled'

Trump's omission left current and former NATO officials reeling, 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."

Nicholas Burns, the US ambassador to NATO under President George W. Bush, said in an interview last month that 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. "Trump is not acting like the leader of the West that all US presidents before him have been dating back to Truman."

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 on Friday said Russia "will think about how to eliminate this threat" if Sweden joins NATO.

"One of Putin's aims for the past 15 years has been to erode the effectiveness of NATO and eventually to undermine it," said Mark Kramer, the program director for the Project on Cold War Studies at Harvard's Davis Center for Russian and Eurasian Studies. "Little did he realize that he would eventually have a US president who would eagerly assist this goal."

"Some of Trump's advisers such as James Mattis and H.R. McMaster are strong supporters of NATO, but Trump himself clearly is not," Kramer continued. "He praises tyrants in Saudi Arabia but can do nothing other than scold democratic allies in NATO."

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.

Kramer added that he did not "rule out" the possibility that Trump had hastened NATO's demise.

"In light of what Trump has said and done, no leader of a NATO country should have any confidence that he would fulfill Article 5 if, say, Estonia were to come under attack from Russia," he said. "Trump's successors might be able to rebuild confidence in the US commitment to NATO, but I don't rule out that the alliance will collapse before Trump leaves office. Putin's goal will have been achieved."