Former President Obama tweeted that "No act of terror will ever change who we are."
President Trump on Monday said that the terrorist attacks on Sept. 11, 2001 exposed America to "the depths of the evil we faced" from extremists.
"On that day not only did the world change, we all changed," he said during a ceremony at the Pentagon in Arlington, Virginia. "Our eyes were opened to the depths of evil we faced."
"But in that hour of darkness, we also came together with a renewed sense of purpose," Trump added during his fist commemoration of the 9/11 massacre as president. "Our differences never looked so small."
Trump also mourned the nearly 3,000 people whose lives were taken "so needlessly" by the terrorists involved in the 9/11 bloodshed.
"We know that not a single day goes by that you don't think of the loved ones who were stolen from this life," he said of those who lost family or friends during the 9/11 attacks.
"Each family here today represents a son or daughter, a sister or brother, a mother or father, that was taken from you on that terrible, terrible day," Trump continued.
"No force on earth can ever take away your memories. Though we can never erase your pain or bring back those you lost, we can honor their sacrifice."
Trump additionally warned terrorists against thinking they could attack the U.S. going forward, vowing "America cannot be intimidated."
"And those who will try will soon join the long list of enemies who dared test our mettle," he said. "We are making plain these savage killers that there is no dark corner beyond our reach."
"America does not bend. We do not waver. And we will never, ever yield...Our values will prevail. And the memory of our loved ones will never, ever fail."
Defense Secretary James Mattis spoke before Trump, meanwhile, and the retired Marine Corps. general blasted the 9/11 terrorists for linking themselves with Islam.
"Maniacs disguised in false religious garb thought they could scare us that day," he said of the al Qaeda hijackers. "Americans are not cotton candy. We are not sea weed drifting in the current."
"Mr. President, your military does not scare," Mattis told Trump. "Your military stands ready and willing to defend this country. We will continue to do so by any means necessary."
President Trump takes part in a 9/11 remembrance ceremony at the Pentagon. Defense Secretary James Mattis and Chairman of the Joint Chiefs of Staff Joseph Dunford also are participating.
Gen. Joseph Dunford, the chairman of the Joint Chiefs of Staff, on Monday vowed that the terrorists behind the 9/11 attacks had not intimidated America into abandoning its values.
"They were attacking symbols that reflect our way of life and values," he said during a ceremony at the Pentagon in Arlington, Virginia. "The terrorists thought they could frighten us to chaos and retreat."
"They were wrong. Instead of hopelessness, we turned to action. Each of us will walk away from this ceremony reminded that the war is not over, and that further sacrifice will be required."
An American flag is unfurled at the Pentagon on the 16th anniversary of 9/11.
A remembrance ceremony is held in New York on the 16th anniversary of the 9/11 attacks.
President Trump leads a moment of silence at the White House.
President Trump on Monday will preside over his first commemoration of the 9/11 terrorist attacks since he entered office.
First lady Melania Trump will join her husband for the solemn, nonpartisan event, with the couple planning on observing a moment of silence at the White House for the victims Monday.
Monday’s remembrance is scheduled for about the same time in the morning a plane struck one of the Twin Towers in New York City on Sept. 11, 2001.
Hijackers then flew another commercial airplane into the other tower, the Pentagon in Arlington, Virginia, and a field near Shanksville, Pennsylvania, killing nearly 3,000 people overall.
The Trumps will also pay their respects at a Pentagon ceremony later Monday led by Defense Secretary Jim Mattis and Gen. Joseph Dunford, chairman of the Joint Chiefs of Staff.
Vice President Mike Pence is slated to represent President Trump’s administration on Monday at an observance ceremony at the 9/11 memorial in Shanksville.
Thousands of 9/11 victims’ relatives, survivors, rescuers and more are expected to gather Monday at the World Trade Center to mourn the deadliest terror attack on U.S. soil.
The 9/11 massacre took place 16 years ago Monday, changing the way Americans viewed and responded to terrorism overseas.
The lives lost have since been honored each year with a number of traditions, including a public recitation of the name of every person who died in the bloodshed.
Sept. 11, 2001, is also commemorated annually with moments of silence and the tolling of bells, and two strong beams of light that shine overnight where the Twin Towers once stood.
The attacks that date were carried out in coordination by 19 hijackers who claimed allegiance to al Qaeda.
The U.S. has remained concerned about radical Islamic terrorism ever since, and Trump has repeatedly vowed to fight the problem during his presidency.
The Associated Press contributed to this report.