In his closed-door meeting with President Vladimir V. Putin in May, Mr. Kerry argued that if Russia and the United States joined forces there was no need for Syria to become another Iraq, invoking an analogy that was calculated to appeal to Russian officials who have long complained that the American intervention there yielded a violent, failed state.

Now he finds himself the face of American intervention, able and willing to be more outspoken than the president he serves. Along with Susan E. Rice, the president’s national security adviser, and Samantha Power, his ambassador to the United Nations, Mr. Kerry has been among those who argued that the chemical weapons attack on Aug. 21 required a strong American response. Gen. Martin E. Dempsey, the chairman of the Joint Chiefs of Staff, has long been skeptical of military involvement in Syria but has never spoken publicly about the current planning.

“The book is still open on John Kerry’s life, but before this, the most famous words he ever uttered were as a Vietnam veteran testifying before the Senate Foreign Relations Committee,” said Jim Gomes, who served as Mr. Kerry’s chief of staff from 1982 to 1986. “His statement today was indicative of someone who believes it would be a mistake not to respond to the horrors in Syria.”

At 69, Mr. Kerry has plunged into his new job with a passion that has surprised many observers. After narrowly losing Ohio and the 2004 presidential race, he no longer harbors dreams of the White House. His State Department perch cannot serve, as it did for Mrs. Clinton, as a potential steppingstone for yet another presidential run. And so for Mr. Kerry, the future is now.

With nearly three decades of service on the Senate Foreign Relations Committee and an extensive array of international contacts, Mr. Kerry does not lack confidence in his own negotiating skills and often meets one on one with foreign leaders. And he has long ties to members of Mr. Obama’s national security team, including Ms. Rice and her predecessor, Tom Donilon.

In his early months on the job, Mr. Kerry talked repeatedly about the need to change Mr. Assad’s “calculation” that he can hang on to power, hinting at a somewhat more robust effort to arm and equip the rebels. At times, that appeared to put him a step ahead of the White House in his public articulation of a strategy, and he eventually eased up on that point.

So when it came time to make the case against Syria, the White House turned twice in the past week to Mr. Kerry, who as a young man building a political career in Massachusetts was a criminal prosecutor. In the ornate Treaty Room of the State Department on Friday, he denounced “the indiscriminate, inconceivable horror of chemical weapons” and, after laying out the intelligence, broadened the case to a moral imperative that “matters deeply to the credibility” of the United States.