"Now we have to steel our resolve to respond," she added.
One day after 49 people were killed when an apparently self-radicalized man, armed with an assault rifle and a handgun, entered a gay nightclub in Florida, Clinton overhauled her schedule to refocus on national security.
An event that was supposed to mark the start of the general-election campaign in battleground Cleveland was transformed into a reflection on the need for patriotism, unity and an effective strategy to combat the Islamic State and its sympathizers.
In light of the Orlando attack, Clinton also postponed two fundraising events that had been scheduled for Monday in Ohio.
Supporters gathered in a warehouse-style room at the Cleveland Industrial Innovation Center and took their seats to more somber acoustic music instead of Clinton's usual pop-heavy playlist. Absent were any Clinton campaign signs; instead, seven American flags adorned the stage.
Her advisers aimed to draw an implicit contrast with her Republican opponent, Donald Trump, in terms of stature and knowledge, even though his name went unmentioned for the duration of her 20-minute remarks.
Before Clinton took the stage, Cleveland Mayor Frank Jackson led the audience in a moment of silence for the victims in Orlando.
While Clinton suggested that the strategy to combat the Islamic State in the Middle East was working, she warned that its ideology was "metastasizing."
She had tough words for countries in the Middle East, including Saudi Arabia and Qatar, that she said funded and fueled terrorism through their support of radical schools and mosques.
"We and our allies must work hand in hand to dismantle the networks that move money and propaganda and arms and fighters across the world," Clinton said.
Clinton took the stage minutes after FBI Director James B. Comey briefed the media on the latest in the investigation. He said the Orlando shooter had been self-radicalized, though it was unclear which terrorist group or ideology motivated him.
Clinton zeroed in on the threat of "lone wolves" and pledged to put together a task force that would bring together government, the private sector and community leaders to combat domestic radicalization.
"The attack in Orlando makes it even more clear: We cannot contain this threat; we must defeat it," Clinton said. "As president, I will make identifying and stopping lone wolves a top priority."
She said local and state law enforcement agencies must be given better, more timely intelligence on potential terrorist threats. She also called for stronger gun-control laws that would prevent suspected terrorists from gaining access to weapons.
"It’s essential that we stop terrorists from getting the tools they need to carry out the attacks," Clinton said. "I believe weapons of war have no place on our streets."
"If the FBI is watching you for a suspected terrorist link, you shouldn’t be able to just go buy a gun with no questions asked," Clinton added. "And yes, if you’re too dangerous to get on a plane, you are too dangerous to buy a gun in America."
Without naming Trump, Clinton denounced "inflammatory anti-Muslim rhetoric" and decried his proposal to ban Muslims from entering the United States, saying that such a policy would make it more difficult to combat terrorism.
"Since 9/11, law enforcement agencies have worked hard to build relationships with Muslim American communities," which "are most likely to recognize the insidious effects of radicalization before it's too late and the best positioned help us block it," Clinton said.
"So we should be intensifying contacts in those communities, not scapegoating or isolating them," she added.
Clinton noted that America is not a country of "winners and losers," a reference to Trump's oft mentioned motto that he hopes to make America "win" again.
"This has always been a country of we, not me," Clinton said. "We stand together because we are stronger together.
"E pluribus unum — out of many, one — has seen us through the darkest chapters of our history," Clinton added.
In the days and weeks after the Sept. 11, 2001, attacks, Clinton noted, Americans came together.
"We did not attack each other. We worked with each other to protect our country and to rebuild," Clinton said. "It is time to get back to the spirit those days — the spirit of 9/12."