In a sombre ceremony held in remembrance of the Sept. 11, 2001 attacks by al Qaeda, U.S. President Joe Biden recalled the nation’s coordinated reaction and vowed to “never give up” in the face of terrorist threats.

In contrast to his recent warnings about perilous divisions in American society, including that some Republicans who support former President Donald Trump’s agenda pose a threat to democracy, Biden’s remarks about national unity on the 21st anniversary of the attacks stood in contrast to his warnings about dangerous divisions in the country.

“I hope we will never forget that we dug deep in the middle of these sad days. We looked out for one another. And we came together,” Biden declared as soldiers lined up behind him, flanking his secretary of defence and senior general.

Al Qaeda hijackers drove four planes into buildings, including the Pentagon in Arlington, Virginia, and the World Trade Center towers in New York, killing over 3,000 people in all.

United Airlines Flight 93’s hijackers were defeated by the passengers, and when the plane landed in a field, no further targets were targeted.

The occasion occurs one year after Biden put an end to the American-led conflict in Afghanistan, which was started two decades ago to hunt down the violent al Qaeda organisation that planned the 9/11 attacks from Afghanistan and carried them out.

Members of both political parties criticised Biden’s disorganised withdrawal of American soldiers last year, which led to the quick collapse of the country to the Taliban.

Biden, however, maintained that the war against terrorism will go on. “We won’t be still. Never will we forget. Never will we give up, he declared.

Ayman al-Zawahiri, an Egyptian surgeon who had a $25 million bounty on his head and assisted in planning the September 11, 2001 attacks, was assassinated by a drone hit in Kabul last month after Biden gave the order.

The Taliban’s ability to provide al Qaeda with safe haven was called into question after Zawahiri’s presence in Kabul was made public.

U.S. military and intelligence officials had previously issued warnings that a complete departure of American forces from Afghanistan may allow both al Qaeda and the Islamic State to acquire power and, if left unchecked, plan against the country in the future.

In his speech, Biden and others emphasised that open-ended military operations and war are not the best ways to deal with the threat posed by terrorism, which has grown over the previous 21 years.

According to Biden, “Our commitment to stop another attack on the United States is without end.”

On Sunday morning, the vice president Kamala Harris and the second gentleman Doug Emhoff attended a ceremony in New York City, while the first lady, Jill Biden, attended one in Pennsylvania.

Key 9/11 moments in New York, such as the times each World Trade Center tower fell, were recognised with a moment of silence following the ringing of a bell.

Families of 9/11 victims have been waiting years for the prosecution and conviction of a number of those accused of plotting and aiding the hijackers, including self-described mastermind Khalid Sheikh Mohammed and four other prisoners held at the U.S. military camp in Guantanamo Bay, Cuba.

To hold the alleged plotters accountable, Biden told reporters on Sunday that “absolutely, there is a plan for that,” but he would not elaborate.