"We had peace talks scheduled a few days ago. I called them off when I learned that they had killed a great American soldier from Puerto Rico and 11 other innocent people," Trump said, regarding the Kabul bomb attack that killed Army Sgt. Elis A. Barreto Ortiz.
"(The Taliban) 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," the president said.
Trump added "And 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 went on to expand upon his threat, saying "They will never have seen anything like what will happen to them. No enemy on earth can match the overwhelming strength skill and might of the American armed forces."
"We do not seek conflict, but if anyone dares to strike our land, we will respond with the full measure of American power and the iron will of the American spirit, and that spirit is unbreakable," Trump said.
Trump's warnings come two days after he pronounced the peace talks that Washington and the Taliban had been pursuing for more than a year in Qatar "dead."
The talks - which had already produced a rough draft of a peace agreement - had been under way with an eye toward putting an end to the almost-two-decade-long war in Afghanistan.
The suspension of negotiations, however, threatens to provoke the Taliban to launch a renewed wave of violence in Afghanistan at an especially delicate moment, just two weeks before that country's presidential election.
Trump went to the Pentagon specifically to remember the 184 people killed there 18 years ago when a commercial jet was rammed into the Defence Department headquarters less than an hour after the Twin Towers in New York received their first impact from another hijacked passenger jet.
It was a day filled with shock, horror, sorrow and righteous fury," Trump told the solemn crowd assembled at the Pentagon for his address.
The president added that he had been watching CNBC when the first plane struck the World Trade Center, saying "I was looking out of a window from a building in midtown Manhattan directly at the World Trade Center when I saw the second plane at a tremendous speed go into the second tower. It was then that I realized the world was going to change."
The president went on to say that "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," echoing a claim he made in July while signing a bill funding medical care for 9/11 first responders.
Although at the time the president had said he spent "a lot of time" at Ground Zero, retired New York Fire Department deputy chief Richard Alles told The New York Times in July that "I spent many months there myself, and I never witnessed him. He was a private citizen at the time. I don't know what kind of role he could have possibly played."