Secretary of State Condoleezza Rice's most senior adviser on Iraq is leaving the State Department to return to his teaching job.

Philip D. Zelikow is the best-known member of Rice's academic brain trust at the State Department, and the author of sometimes contrarian appraisals of the Iraq conflict and reconstruction effort. He holds the title counselor, a sort of adviser without portfolio.

In a resignation letter dated Monday, Zelikow said he will return to teaching at the University of Virginia in January. He cited a "long-standing debt to my family" and "truly riveting obligation to college bursars," for his children's tuition.

Zelikow was among the first people Rice hired after she took over as secretary of state in 2005. She also brought in other fellow academics to join a team of Republican political strategists to be her top advisers. His first assignment was a scouting trip to Iraq.

When Zelikow returned, according to the Bob Woodward book "State of Denial," he wrote a secret memo characterizing Iraq as "a failed state" two years after the U.S.-led invasion. In September 2005, he wrote a memo estimating a 70 percent chance of success in achieving a stable, democratic Iraq, and what he called a "significant risk" of "catastrophic failure," the book said.

Besides his internal assessments of Iraq, Zelikow's main duties have included work on the U.S. plan to ship civilian nuclear fuel and technology to India. The plan, which the Senate overwhelmingly endorsed last week, reverses decades of U.S. anti-proliferation policy. The Bush administration says it strengthens a key relationship with a friendly Asian power that has long maintained what the United States considers a responsible nuclear program.

Rice and Zelikow had written a book together before she took a leave from Stanford to be President Bush's first-term national security adviser. Zelikow took a leave from U.Va. to be the top staffer on the Sept. 11 study commission before joining Rice's staff.

His name has been mentioned as a possible candidate to succeed John Bolton as U.N. ambassador, but a U.S. official said that is not likely. The official spoke on condition of anonymity because the White House hopes the Senate will confirm Bolton, whose recess appointment runs out in January.