President Biden traveled to the Pentagon on Sunday to mark the 21-year anniversary of the Sept. 11 attacks, remembering the attack’s victims while vowing to stand up for democracy in their honor.

“Here at the Pentagon, which was both the scene of the horrific terrorist attack and the command center for our response to defend and protect the American people, so many heroes were made here,” Biden said.

“So many of your loved ones were those heroes,” he said, noting civilians and service members who immediately leapt into action.

Biden, who served as a senator at the time of the attacks, said he remembers seeing the smoke from the Pentagon that day as he returned to his office.

The president during the speech also honored veterans who served in wars following the attacks in places like Afghanistan and Iraq.

“We owe you,” Biden said. “We owe you an incredible, an incredible debt. A debt that can never be repaid, but we’ll never fail to meet the sacred obligation to you.”

He also referenced the killings of al Qaeda leader Osama bin Laden in 2011 and his successor, Ayman al-Zawahiri, at the end of July, saying the United States “will not rest.”

Al-Zawahiri’s killing came almost a year after the United States withdrew its military presence in Afghanistan as the Taliban retook control of the country. 

Biden has touted the operation as proof the United States can still fight terrorism in Afghanistan without troops on the ground, a sentiment he echoed on Sunday.

“We’ll continue to monitor and disrupt those terrorist activities wherever we find them, wherever they exist, and we will never hesitate to do what’s necessary to defend the American people,” he said.

The president also leveraged the anniversary to speak about democracy, a topic he has increasingly referenced as the midterm elections approach.

“We have an obligation, a duty, a responsibility to defend, preserve and protect our democracy,” Biden said. “The very democracy that guarantees the rights of freedom that those terrorists on 9/11 sought to bury in the burning fire, smoke and ash.”

Biden has cast former President Trump and “MAGA Republicans” as threats to the republic in recent speeches. But on Sunday, he vowed to maintain America’s democratic system while not directly attacking Trump.

“It’s not enough to stand up for democracy once a year or every now and then,” Biden said. “It’s something we have to do every single day. So this is a day not only to remember, but a day of renewal and resolve for each and every American.”

–Updated at 11:11 a.m.