The chief of NATO has paid tribute to those killed in the 9/11 attacks 18 years ago and to the Alliance's troops killed in Afghanistan in the years after.
Speaking next to a twisted piece of metal from the Twin Towers outside NATO headquarters in Brussels, Mr Stoltenberg said the Alliance must make sure the sacrifices of its soldiers were not in vain.
"Many have paid the ultimate price. And many more have been seriously wounded," Mr Stoltenberg said.
"We remember them. And we must make sure that their sacrifice was not in vain.
"The fight against terrorism is a global, generational fight. We remain committed to Afghanistan.
"NATO allies and partners will continue to train and advise Afghan security forces. To make them stronger, so that they can fight international terrorism and create security and stability in their own country.
"Our military presence is there to create the conditions for peace. But the road to peace is long and hard."
The attack on 11 September 2001, which killed 2,9777 people, was the first and only time in NATO's history that the Alliance invoked its collective defence clause, Article 5.
"One for all, and all for one," said Mr Stoltenberg in his speech.
Allied troops, including the UK, fought alongside US forces in Afghanistan in response to 9/11.
Speaking at a conference in the UK, the Defence Secretary Ben Wallace also marked the anniversary.
"For many, it [9/11] was the first awakening of the full horror of global terrorism," Mr Wallace said.
"But it was because of our investment in strength in defence, because of the strength of our capability that the UK was able to fight alongside our allies."
Meanwhile, the Red Arrows have also paid their respects to the victims of 9/11.
The RAF's display team, who are currently touring North America, released pictures of their crew on a recent visit to the 9/11 memorial in New York City with the words: "We will never forget".