Vowing the mistakes of Iraq will not be repeated, Kerry said evidence gathered by the intelligence community shows 1,429 people, including 400 children, were killed in an Aug. 21 chemical weapons attack, and the attack was launched by forces loyal to the Assad regime.
"We know where the rockets were launched from, and at what time," Kerry said, labeling Assad a "murderer."
"We know where they landed, and when. We know rockets came only from regime-controlled areas and went only to opposition-controlled or contested neighborhoods. ...
"This is the indiscriminate, inconceivable horror of chemical weapons. This is what Assad did to his own people. The only question now, he said, is what the United States will do. ...
"Our choices then, in history, had great consequences. And our choice today has great consequences," Kerry said, noting that after World War I the international community vowed to eradicate the use of chemical weapons.
Kerry set the stage for a U.S. response, saying other nations are waiting to see if Washington will stand by its word to deter other regimes from weapons programs that could endanger the world.
"We are the United States of America. We are the country that has tried, not always successfully, but always tried to honor a set of universal values around which we have organized our lives and our aspirations," he said.
"This crime against conscience, this crime against humanity, this crime against the most fundamental principles of international community, against the norm of the international community, this matters to us.
"And it matters to who we are. And it matters to leadership and to our credibility in the world.
"My friends, it matters here if nothing is done. It matters if the world speaks out in condemnation and then nothing happens. ...
"We are not alone in our condemnation and we are not alone in our will to act."
Jordana Brewster on Paul Walker: 'He was an enormous presence in my life'
Exploding whale video goes viral on Internet