The greatest lesson from the Sept 11, 2001, attacks on the United States was the value of national unity, said President Joe Biden, as the country yesterday marked the 20th anniversary of the single largest loss of life from a foreign attack on American soil.
"We learnt that unity is the one thing that must never break. Unity is what makes us who we are," Mr Biden said in his pre-taped video message.
"That is the central lesson… that at our most vulnerable, in the push and pull of all that makes us human, in the battle for the soul of America, unity is our greatest strength," he said.
"We are unique in the history of the world because we are the only nation based on an idea… that everyone is created equal and should be treated equally throughout their lives. That is the task before us. To once again, lead not just by the example of our power, but by the power of our example."
Sombre ceremonies in New York City, Arlington in Virginia and Shanksville in Pennsylvania marked the 20th anniversary of the terrorist attacks that killed 2,977 people from 90 nations.
Mr Biden visited all three sites, starting with New York City, where he was joined by former presidents Bill Clinton and Barack Obama, among others. They observed a moment of silence at 8.46am (8.46pm Singapore time yesterday) to mark the time the first plane hit.
In his video message on the eve of the anniversary, Mr Biden said the US witnessed not only heroism, but also "the darker forces of human nature".
"Fear and anger, resentment and violence against Muslim Americans, true and faithful followers of a peaceful religion. We saw a national unity bend," he said.
That day in 2001, 19 militants linked to the Al-Qaeda terror group hijacked four commercial airplanes. Two were flown into the Twin Towers of New York's World Trade Center, which collapsed. A third was flown into the Pentagon in Arlington. Crew members and passengers on a fourth plane, which was turned to head for the Capitol in Washington DC, fought with the hijackers; the plane crashed in a field near Shanksville.
Several family members of the victims in New York spoke at the ceremony in the city, their voices often cracking with emotion.
Mr Biden said in his message: "No matter how much time has passed, these commemorations bring everything painfully back."
America would also "never stop" hunting down terrorists who sought to harm the nation, he warned.
The anniversary of the attacks that sparked two wars - with the US invading Afghanistan in 2001 and Iraq in 2003 - was marked elsewhere too.
British Prime Minister Boris Johnson said: "We can now say with the perspective of 20 years that (the attackers) failed to shake our belief in freedom and democracy."
Australian Prime Minister Scott Morrison wrote in an opinion piece: "Sept 11 reminded us that freedom is always fragile... That day was an attack on free peoples everywhere... The terrorists ultimately failed in their attempts to crush our resolve and change our way of life."
In an opinion piece in The Straits Times, Singapore Prime Minister Lee Hsien Loong wrote: "Our world changed overnight.
"We discovered right here among us a terrorist group having a common ideology and direct links with Al-Qaeda in Afghanistan - the Jemaah Islamiah group." Its plans for attacks in Singapore were swiftly disrupted, he noted. And "in an existential crisis, Singaporeans instinctively pulled together, and responded strongly and cohesively to keep ourselves safe".
Mr Lee also wrote on Facebook: "Let us resolve to fortify ourselves so that should we ever face another such test one day, we will come through again, stronger, as one united people."
In The Straits Times, he added: "Now that the US has left Afghanistan, we will have to watch closely how the situation there develops, whether groups based in Afghanistan will again threaten our security, and where else new fronts of terrorism may emerge."