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 Bush's pragmatic security advisors, including Brent Scowcroft, Gates was part of that inner circle. He remains close not only to Scowcroft but to other Rumsfeld rivals as well, 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 Wednesday, hoping for a less combative Pentagon chief. But he has proved controversial in the past and 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, 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 both men. "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, Bush praised Gates' experience but also made clear that he expected the nominee to find fresh approaches in Iraq that seemed to elude Rumsfeld's more insular team.
"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 H. Hamilton (D-Ind.), which has been charged with developing an array of alternatives to Bush's 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.
If confirmed, Gates will be in charge of finding a way out of a war that he might never have started in the first place.
Indeed, whereas Rumsfeld was a leading advocate for invading Iraq after the Sept. 11 attacks — part of an ideological team that regarded deposing Saddam Hussein as unfinished business — Gates was among those who had cautioned the first President Bush about the perils of pressing toward Baghdad after expelling the Iraqi army from Kuwait in 1991.
Gates, 63, started 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 Bill Clinton was elected president.
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 withdrew amid congressional opposition. Gates was considered too closely tied to the Iran-Contra scandal, which had involved 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 lasting connections with Scowcroft and Rice.
After the Iran-Contra scandal had faded, Gates was again nominated to be CIA director, this time by the elder Bush. His 1991 confirmation turned into a battle as well. 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. He was later accused of failing to anticipate that communist regime's collapse.
Gates was confirmed, but his margin of approval — 64 to 31 — was the largest expression of Senate opposition to an intelligence director nominee.
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 of its recommendations.
While CIA director, Gates became embroiled in a controversy over an inquiry into questionable financial dealings between a U.S. branch of an Italian-owned bank and the Iraqi government in the years before the 1991 Persian Gulf War. The Senate Intelligence Committee faulted CIA and Justice Department officials but found no criminal wrongdoing.