Standing in front of a twisted piece of metal from the North Tower of the World Trade Center, NATO Secretary-General Jens Stoltenberg said the 29 member alliance was "strong" on the 18th anniversary of September 11, 2001.
"On 9/11, terrorists destroyed the Twin Towers, but they will never destroy our way of life, our free and open societies and the unbreakable bond between North America and Europe," Stoltenberg said.
"Our alliance keeps us strong and it keeps us safe."
Stoltenberg and US ambassador to NATO Kay Bailey Hutchison placed a wreath on the Article 5 memorial at NATO's headquarters in Brussels and spoke about the alliance in remarks at a ceremony in honour of the nearly 3,000 victims of the coordinated terror attacks in New York and Washington.
The attacks invoked the first and only use of NATO's Article 5 which states that "an armed attack against one or more of them in Europe or North America shall be considered an attack against them all".
Ambassador Hutchison said at the ceremony: "9/11 was not just an attack on America. It was not just an attack on 3,000 innocent citizens. It was an attack on who we are."
"There will always be people who want to destroy our values... as long as we remain united we will always defeat the minions of tyranny," Ambassador Hutchison said.
NATO allies would support the US in Afghanistan starting in late 2001, eventually, taking charge of the security assistance mission to train Afghan forces in 2003.
Stoltenberg said during his remarks that NATO must remain committed to Afghanistan but that the "road to peace is long and hard".
He also remembered those who have died and said: "we must make sure that the sacrifice was not in vain."
The NATO alliance has been tested since the election of US President Donald Trump who had previously bemoaned European defence contributions as too little, stating that the US was paying for "Europe's protection".
In a February 2019 Harvard Kennedy School report on NATO, two former US NATO envoys Nicholas Burns and Douglas Lute wrote that the "single greatest challenge" for the alliance "is the absence of strong, principled American presidential leadership for the first time in its history."