President Donald Trump on Wednesday warned terrorists not to return to the United States, during a 9/11 commemoration ceremony at the Pentagon.

The president acknowledged in his commemoration speech that he had peace talks scheduled with the Taliban at Camp David, but called them off after they took responsibility for a suicide bombing in Kabul that killed 12 people, including one American soldier.

“They thought they would use this attack to show strength, but actually what they showed is an unrelenting weakness,” Trump said.

He said that in the days since the attack in Kabul, America had escalated attacks against the Taliban as he would continue to fight terrorism.

“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,” the president said.

He clarified that he was not talking about nuclear arms, but vowed retaliation.

“They will never have seen anything like what will happen to them,” he warned. “No enemy on earth can match the overwhelming strength skill and might of the American armed forces.”

Watch the entire event below:

The president also saluted the heroes of the military who continued to fight terrorism overseas.

“They have crossed seas, climbed mountains, trekked through deserts, and rushed into enemy compounds to face down the threat of radical Islamic terrorism,” he said.

He recalled the nearly 7,000 service members who have died in the fight against terrorism.

“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,” he said.