The president stood and read a plaque at the memorial before touching it briefly and stepping back to gather his thoughts

Joe Biden
Joe Biden at the 9/11 Memorial at NATO headquarters

President Joe Biden took an unannounced break from meetings during his international trip to pay a visit on Monday to the 9/11 memorial at NATO headquarters in Brussels.

The memorial is comprised of a piece of twisted steel from the World Trade Center, which collapsed after the terrorist attacks of Sept. 11, 2001.

As Associated Press reporter Jonathan Lemire noted in a tweet, NATO's mutual defense pact, Article 5, was invoked for the first time in history less than 24 hours after the attacks.

According to reporters traveling with him, the president stood and read the plaque at the memorial before touching it briefly and stepping back to gather his thoughts.

Biden, 78, spoke earlier on Monday about the need to remind Americans that NATO allies came to the aid of the U.S. following the events of Sept. 11, according to reporters.

The NATO memorial was unveiled in 2017, with then-President Donald Trump delivering remarks at the ceremony in which he noted the significance of NATO's decisive action in the wake of the attacks.

"This ceremony is a day for both remembrance and resolve. We remember and mourn those nearly 3,000 innocent people who were brutally murdered by terrorists on September 11th, 2001," Trump said. "Our NATO allies responded swiftly and decisively, invoking for the first time in its history the Article 5 collective defense commitments."

Joe Biden
Joe Biden at the 9/11 Memorial at NATO headquarters

Article 5 is a key section of the North Atlantic Treaty and commits each member state "to consider an armed attack against one member state, in Europe or North America, to be an armed attack against them all."

Biden is in Belgium for his first NATO summit as president after wrapping up a three-day gathering of the Group of Seven and a meeting with Queen Elizabeth in the U.K.

As National Security Advisor Jake Sullivan told reporters ahead of Biden's visit on Monday, the president's "overriding objective" at NATO was "of sending a clear message to Allies and adversaries alike that Article 5 is a sacred guarantee; that the United States regards NATO as the foundation for our security - not just in the Euro Atlantic, but worldwide - and that we will be there for our Allies."

"We will have their backs just as they've had our backs," Sullivan added.

Biden said in a statement on Twitter that the Monday visit to NATO headquarters offered a message of "reaffirming America's commitment to our 29 Allies and our vision for a more secure future."

September 2020 marked the 19th anniversary of the attacks, with then-candidate Biden attending a memorial ceremony in New York City to honor the lives lost.

"I'm not going to make any news today," the former vice president told reporters. "I'm not going to talk about anything other than 9/11. We took all our advertising down. It's a solemn day. That's how we're going to keep it, okay? You can determine whether I make news but I'm not going to be holding any press conferences."