Whereas Rumsfeld often seemed bent on running roughshod over the Pentagon brass, Gates is described by longtime associates as collegial and a consensus-builder.

If Rumsfeld had little regard for President George H.W. Bush and many of his pragmatic security advisers, including Brent Scowcroft, Gates was part of that inner circle. He remains close not only to Scowcroft but to other Rumsfeld rivals, including Secretary of State Condoleezza Rice.

Rumsfeld placed little trust in intelligence agencies and pushed the military to encroach on their turf. In a turning of the tables, a 27-year veteran of the CIA and the National Security Council is poised to take charge of the military.

Democrats praised Gates' nomination, hoping for a less combative Pentagon chief. But Gates has proved controversial in the past. He was forced to withdraw from his first nomination as CIA director before winning a split-vote confirmation four years later.

Across the national-security community today, the deep contrasts between Rumsfeld and Gates were a subject of conversation.

Rumsfeld "is a guy who is kind of burdened with his own certitude at times," said John Gannon, a former high-ranking CIA official who worked with Rumsfeld and Gates. "That is not Bob Gates. He came out of an analytic culture where listening to the ideas of others and questioning your own assumptions is part of the tradecraft."

In announcing the changes today, Bush praised Gates' experience, but he also made clear that he expected the nominee to find fresh approaches in Iraq.

"He's a steady, solid leader who can help make the necessary adjustments in our approach to meet our current challenges," Bush said.

Gates is a member of a White House panel, led by former Secretary of State James A. Baker III and former Rep. Lee Hamilton, D-Ind., that has been charged with developing an array of alternatives to foundering Iraq policies. Gates recently traveled to Iraq as part of that team, according to the White House, and met with Iraqi leaders and U.S. military commanders.

Whereas Rumsfeld was a leading advocate for invading Iraq in the aftermath of the Sept. 11 terrorist attacks — part of an ideological team that regarded deposing Saddam Hussein as unfinished business — Gates was among those who cautioned the first President Bush not to press toward Baghdad after expelling the Iraqi army from Kuwait in 1992.

Gates, 63, started out at the CIA fresh out of college as a Soviet analyst and is the only agency officer to rise through the analytic ranks to take the top job. He served as CIA director under the elder Bush for a little more than a year before President Clinton was elected.

Gates, who has been president of Texas A&M University since 2002, is not without detractors. He was first nominated to be CIA director by President Reagan in 1987, but he withdrew amid congressional opposition. Gates was considered too closely tied to the Iran-Contra scandal, which had been engineered by former CIA Director William J. Casey while Gates was his deputy.

Rather than taking the helm at the CIA, Gates joined the National Security Council staff at the White House, where he made connections with Scowcroft and Rice.

After the Iran-Contra scandal faded, Gates was nominated to be CIA director by President George H.W. Bush. His 1991 confirmation turned into a battle. Gates was accused of politicizing intelligence — a charge that has been repeatedly aimed at the White House of George W. Bush. Agency analysts testified that he was heavy-handed in seeking to influence assessments of the Soviet Union. Helater was accused of failing to anticipate that communist regime's collapse.

Gates was confirmed, but his margin of approval — 64-31 — represented the largest expression of Senate opposition to a nominee for intelligence director.

One of Gates' first initiatives as director was to confront perceptions that intelligence had been politicized within the agency. He appointed a task force on "analytic objectivity" and implemented all its recommendations.

While CIA director, Gates became embroiled in a controversy over an investigation of questionable financial dealings between a U.S. branch of an Italian government-owned bank and the Iraqi government before the 1991 Persian Gulf War. The Senate Intelligence Committee faulted CIA and Justice Department officials but found no criminal wrongdoing.