President Trump brought up his canceled Camp David meeting with the Taliban during his speech at the Pentagon Wednesday commemorating the Sept, 11, 2001 terrorist attacks.

That such a secret meeting was supposed to take place at Camp David in the first place rattled the president's critics and even some of his allies. Mr. Trump and his administration have been defending both the move to set up the meeting and to cancel it since.

"I called them off when I learned that they had killed a great American soldier from Puerto Rico and 11 other innocent people," the president said. "They thought they would use this attack to show strength, but actually, what they showed, is unrelenting weakness. The last four days, we have hit our enemy harder than they have ever been hit before and that will continue."

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He added that anyone who dares strike the U.S. will be dealt with gravely.

"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 even talking about nuclear power," he said. "They will never have seen anything like what will happen to them."

Mr. Trump also recalled where he was on that fateful day 18 years ago, sitting at home watching a business television show when "all of the sudden," the screen cut away. The president described the "great confusion," until he watched as a second plane came at "tremendous speed" and crashed into the second World Trade Center Tower.

"It was then I realized the world was going to change," he said.

The president said that for every American who lived through that day, "the September 11 attack is seared into our soul."