President Trump Donald John TrumpFederal prosecutor speaks out, says Barr 'has brought shame' on Justice Dept. Former Pence aide: White House staffers discussed Trump refusing to leave office Progressive group buys domain name of Trump's No. 1 Supreme Court pick MORE on Wednesday marked the 18th anniversary of the Sept. 11, 2001, terror attacks with a moment of silence and a ceremony at the Pentagon while delivering a warning to the Taliban.

During the morning's ceremonies, Trump acknowledged a breakdown in recent peace efforts with the Taliban in Afghanistan and vowed to unleash unprecedented power should the group strike the United States again.

Trump said he called off the Camp David meeting planned for over the weekend when he learned the Taliban carried out an attack that killed a “great American soldier from Puerto Rico,” referring to a suicide bombing in Kabul last week claimed by the militant group.

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“They thought they would use this attack to show strength, but what they actually showed is unrelenting weakness,” Trump said in remarks at the Pentagon, adding that the U.S. has “hit our enemy harder than they have ever been hit before” in the four days since the canceled meeting.

“If for any reason they come back to our country, we will go wherever they are and use power the likes of which the United States has never used before — and I’m not talking about nuclear power — they will never have seen anything like what will happen to them,” Trump said.

The president’s reference to the ongoing war in Afghanistan, which was triggered by the 9/11 attacks, came amid a 20 minute speech outside the Pentagon, where a hijacked plane crashed and killed 184 people nearly two decades ago.

Trump praised American firefighters, military members, first responders and everyday civilians for their brave efforts to save victims of the 9/11 attacks. Trump mentioned several first responders by name, one of whom, Rick Rescorla, Trump said he would soon award the Presidential Citizens Medal posthumously.

The president was joined by several top advisers and members of his Cabinet, including Defense Secretary Mark Esper Mark EsperOvernight Defense: Stopgap spending measure awaits Senate vote | Trump nominates former Nunes aide for intelligence community watchdog | Trump extends ban on racial discrimination training to contractors, military Overnight Defense: Pentagon redirects pandemic funding to defense contractors | US planning for full Afghanistan withdrawal by May | Anti-Trump GOP group puts ads in military papers Official: Pentagon has started 'prudent planning' for full Afghanistan withdrawal by May MORE who also delivered remarks, as well as U.S. military leaders.

A native New Yorker, Trump recounted when he first learned of the attacks on the World Trade Center. He said he was watching a “major business television show” that morning when it cut away. He recalled the confusion over the cause of the smoke billowing from the first tower when another plane crashed into the second tower “with tremendous speed.”

“It was then that I realized the world was going to change. I was no longer going to be, and it could never ever be that innocent place that I thought it was,” he said. “Soon after, I went down to ground zero with men who worked for me to try to help in any little way that we could. We were not alone."

Trump and first lady Melania Trump Melania TrumpWatchdog confirms State Dept. canceled award for journalist who criticized Trump Ginsburg becomes the first woman to lie in state in the Capitol Rabbi memorializes Ginsburg: Her dissents were 'blueprints for the future' MORE participated in a moment of silence on the South Lawn of the White House at 8:46 a.m. in recognition of the moment the first plane, American Airlines Flight 11, flew into the North Tower of the World Trade Center in New York City.

They were joined by families of victims of the attacks, survivors and former law enforcement officials.

The president then traveled to the Pentagon to participate in a wreath-laying ceremony and to deliver remarks.

Leaders around Washington took part in ceremonies to commemorate the anniversary of the attacks.

Speaker Nancy Pelosi Nancy PelosiDemocratic senator to party: 'A little message discipline wouldn't kill us' Overnight Health Care: New wave of COVID-19 cases builds in US | Florida to lift all coronavirus restrictions on restaurants, bars | Trump stirs questions with 0 drug coupon plan Overnight Defense: Appeals court revives House lawsuit against military funding for border wall | Dems push for limits on transferring military gear to police | Lawmakers ask for IG probe into Pentagon's use of COVID-19 funds MORE (D-Calif.) and House Minority Leader Kevin McCarthy Kevin Owen McCarthyHouse to vote on resolution affirming peaceful transition of power Ginsburg becomes the first woman to lie in state in the Capitol McCarthy says there will be a peaceful transition if Biden wins MORE (R-Calif.) led a moment of silence for members of Congress at the Capitol at 8:46 a.m.

Acting Homeland Security Secretary Kevin McAleenan attended a ceremony in New York City alongside current and former local leaders, where they read the names of those killed in the attacks. McAleenan honored Secret Service officer Craig Miller, who was killed 18 years ago.

The president has in the past two years marked 9/11 by attending ceremonies at the Pentagon and in Shanksville, Pa., at the site where United Flight 93 crashed.

Earlier this year, Trump signed a bill reauthorizing the victims’ fund to assist first responders at the scene of the attacks.

Trump has made controversial remarks in the past about 9/11, such as when he made the baseless claim that there were Muslims in New Jersey cheering the fall of the Twin Towers and when he appeared to blame former President Clinton for the attacks by failing to kill Osama bin Laden.

Trump preceded his attendance at this year’s Pentagon ceremony with a series of tweets lashing out at the Federal Reserve and new polling from The Washington Post and ABC News that showed him trailing several Democratic presidential candidates in hypothetical 2020 matchups.

“If it weren’t for the never ending Fake News about me, and with all that I have done (more than any other President in the first 2 1/2 years!), I would be leading the ‘Partners’ of the LameStream Media by 20 points. Sorry, but true!” Trump tweeted at 8:19 a.m.

Two minutes later, he returned the focus to the day’s more solemn events.

“Leaving the White House soon to speak at the Pentagon,” he tweeted at 8:21 a.m. “My great honor!”