'America cannot be intimidated, and those who try will soon join the long list of vanquished enemies,' president says.
“The terrorists who attacked us thought they could incite fear and weaken our spirit, but America cannot be intimidated, and those who try will soon join the long list of vanquished enemies who dared to test our mettle,” Trump said at a ceremony at the Pentagon.
"America does not bend, we do not waver, and we will never, ever yield,” he added.
Prior to the ceremony, he observed a moment of silence with first lady Melania Trump at the White House.
The U.S. was hit by the deadliest attack on its soil when al-Qaeda operatives flew planes into the World Trade Center in New York's, and the Pentagon in northern Virginia. A fourth plane crashed in Pennsylvania when passengers attempted to overtake hijackers.
That plane was likely bound for either the White House or U.S. Capitol.
In all, nearly 3,000 victims were killed.
Trump vowed that those who died did not pass in vain, telling their surviving loved ones the nation mourns with them.
“The living, breathing soul of America wept with grief for every life taken on that day,” Trump said. "Though we can never erase your pain, or bring back those you lost, we can honor their sacrifice by pledging our resolve to do whatever we must to keep our people safe."
Speaking later at the site of the crashed fourth plane, United Flight 93, Vice President Mike Pence recalled the day when he was a member of Congress and was told the plane was 12 minutes from the Capitol.
"It was the longest 12 minutes of my life," he said at a remembrance ceremony in Shanksville, Pennsylvania. "But it turned to 13 minutes, then 14 and then we were informed that the plane had gone down in a field in Pennsylvania."
In New York, at least 1,000 people gathered at the 9/11 memorial in lower Manhattan for a somber ceremony as bells tolled during a moment of silence at 8.46 a.m. (1246GMT), the time when American Airlines Flight 11 slammed into the World Trade Center's north tower.
A second bell tolled at 9.03 a.m. (1303GMT) to mark the moment United Airlines Flight 175 hit the South Tower. The toll was followed by four others to mark the Pentagon attack, the Shanksville crash and the times the north and south towers fell.