This article is by Alan Feuer, John Eligon and William K. Rashbaum.

Cyrus R. Vance Jr., the Manhattan district attorney, seemed preoccupied when he sat down with two reporters last Monday. He already knew what the world would soon learn: his marquee prosecution, the sexual assault case against Dominique Strauss-Kahn, was falling apart. Privately, his aides had told him they had discovered grave problems with the accuser’s credibility.

As the interview began, but before Mr. Vance was asked a question, he offered an unsolicited defense — not just of the Strauss-Kahn case, but of his overall stewardship. “Ultimately,” he said, “the success of a D.A.’s office, and of a D.A., is measured not in individual cases, but over time.”

“The cases you don’t read about,” he added, “define what the job of a D.A. really is.”

But that job has grown increasingly tumultuous. Since Mr. Vance took over 18 months ago, morale in some parts of the office has begun to sag, in part because of his firing of some prosecutors. Relations with one of the office’s key partners, the Police Department, have grown tense at times, with the agencies competing over many issues, including control of anticrime initiatives, officials on both sides say.

Mr. Vance’s predecessor, Robert M. Morgenthau, who became the pre-eminent district attorney in the country while holding the post for 35 years, was once a close ally of Mr. Vance’s, providing crucial support for his election in 2009. Mr. Vance worked for Mr. Morgenthau in the 1980s.