Americans will always remember Sept. 11, 2001. Fifteen years later, the feelings of confusion, shock and loss from the deadly attack on our country are still vivid. But the next day, Sept. 12, 2001, was also historic. That was the day America’s staunchest allies stood with us in solidarity, ready to respond in America’s time of need. It was the day when our then 18 NATO allies invoked, for the first and only time in history, Article 5 of the Washington Treaty, the clause that declares that an attack on one nation is an attack on all….

R. Nicholas Burns, the United States ambassador to NATO at the time, recalled the moment when the leaders of the alliance stood together in the Brussels headquarters and unanimously declared that America had suffered an armed attack and that all were prepared to provide assistance. “When we needed allies the most, they were there for us,” Mr. Burns said. “The invocation of Article 5 demonstrated the power of the collective, versus the strength of one country trying to stand alone….”

[T]hroughout the past nearly seven decades, the commitment to Article 5 remains at NATO’s foundation….

Each ally knows that if attacked, it will not stand alone — for all 28 allies, that is a commitment worth investing in….

Friends prove their friendship in times of need. Very early on the morning of Sept. 12, 2001, a senior American officer was surprised to find an Italian military officer on duty in full uniform at his desk at NATO headquarters, which had been evacuated because of an assessed threat. When asked why he was there, the Italian officer snapped to attention. “Sir, your country saved Europe twice during the last century,” he told the American. “The least I can do on this day is to be on duty.”

Later that day, 24 hours after the Sept. 11 attacks, our allies formally proved their commitment by invoking Article 5. Just as America had stood with Europe through two world wars, the Marshall Plan, the Cold War and beyond, on our darkest day Americans did not stand alone. Condoleezza Rice, then the national security adviser, wrote in her autobiography that when Mr. Burns called to give her the news, the White House was still trying to understand who was responsible and what we should do next. As she hung up the phone she thought to herself, “It’s really good to have friends.”

Douglas Lute is the United States ambassador to NATO.

Image: NATO's decision on September 12 was historic (graphic: NATO)