In a twist of fate, the once hero of the anti-war movement has in past days become the very public voice of calls by the US administration to punish the regime of Syrian President Bashar al-Assad for what Washington says is its unquestionable use of chemical weapons.
Back in 1971 fresh from the horrors of the war in Southeast Asia where he served as a naval lieutenant, Kerry led a group of Vietnam Veterans Against the War to testify before emotional congressional hearings about the conflict.
"How do you ask a man to be the last man to die in Vietnam? How do you ask a man to be the last man to die for a mistake?" he agonized.
Yet on Tuesday, the now silver-haired Kerry, who turns 70 in December, was back in front of the committee in his role as secretary of state with a fresh mission -- to persuade lawmakers to fire missiles at Syrian targets.
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Adolf Hitler, late Iraqi dictator Saddam Hussein, Assad and the leader of a Japanese cult were the only figures in history so far to use sarin gas, he said.
"History is full of opportunity of moments where someone didn't stand up and act when it made a difference," the top US diplomat said.
"If the United States... Knowing that we've drawn a line that the world has drawn with us, is unwilling to stand up and confront that, it is an absolute certainty that gas will proliferate," he warned.
Sitting next to him was Defense Secretary Chuck Hagel, also a Vietnam vet, while from the benches opposite they were grilled by Republican Senator John McCain, who was imprisoned and tortured in the jail dubbed the Hanoi Hilton.
"There's not one of us who doesn't understand what going to war means, and we don't want to go to war. We don't believe we are going to war in the classic sense of taking American troops and America to war," Kerry said.