WASHINGTON » On his 10th day on the job, Defense Secretary Robert M. Gates signed off on an ambitious if politically charged plan to build a new missile shield in Europe. Just two weeks later, he supported an even more wrenching decision to send additional U.S. troops to Iraq, into a war that was not going well.

That was nearly three years, one president and a political lifetime ago. Now serving Barack Obama instead of George W. Bush, Gates just recommended jettisoning his own missile defense program in favor of a reformulated version and once again is wrestling with whether to send more troops abroad, in this case to Afghanistan.

Quiet and unassuming, Gates has emerged as the man in the middle between policies of the past he once championed and the revisions and reversals he is now carrying out. His stature and credibility have allowed him to extract concessions on the inside, including on missile defense, according to senior officials, while serving as a powerful shield against Republican spears on the outside.

Along the way, Gates has become a White House favorite, both for his pragmatic style and his political value. With little national security experience of his own, Obama has leaned heavily on the holdover Pentagon chief for advice, aides said. And as a result, Gates has played a central role in reshaping national security policy, including fixing a broken Pentagon procurement system to recalibrating the size of the country's nuclear arsenal.

“;The president values what Secretary Gates says — and not just values, he knows what he brings to the table is 30 years of experience in Democratic and Republican administrations,”; said Rahm Emanuel, the White House chief of staff. “;He understands that none of these decisions are between good and bad but between bad and worse.”;

The looming decision on Afghanistan could put Gates' experience to the test as never before. With both Gen. Stanley A. McChrystal, the top U.S. commander, and Adm. Mike Mullen, chairman of the Joint Chiefs of Staff, now on record as favoring more combat troops, Gates must balance his commanders' desires and his president's apparent skepticism.

Gates has made the transition from the Bush years to the Obama administration with insider skills honed over decades of working for presidents of both parties. He reached out from the start to Secretary of State Hillary Rodham Clinton to encourage more civilian roles in Afghanistan and Iraq, and teamed up with Emanuel to kill the F-22 fighter program.

Just as he was in the Bush Cabinet, he has at times been caught between high-powered hawk and dove figures. When Obama sent more troops to Afghanistan this year, Gates maneuvered between Clinton, who strongly favored the reinforcement, and Vice President Joseph R. Biden Jr., who resisted it. And he has been a voice of caution on issues like Obama's desire to eventually eliminate all nuclear weapons.

For Republicans, Gates poses a quandary in assessing Obama's national security decisions: do they look at him as a turncoat for dismantling some of Bush's policies or as the best hope for moderating changes brought by a Democratic administration?

“;He's got a president who's pushing in a different direction than the previous president and he's got to deal with that,”; said Peter H. Wehner, a White House strategic adviser to Bush. “;For us in the Bush administration, he's got a lot of money in the bank because of Iraq and the surge.”;

Wehner recalled a conversation over the weekend with fellow conservatives about the missile defense decision. “;Nobody said anything nasty or vicious about him,”; he said. “;There was genuine puzzlement.”;

Gates's shifting role can be summed up in terms familiar to the defense secretary, an avid film buff who routinely brings piles of DVDs on long trips and cites favorite movies in conversation to make a point.

In his new memoir, Matt Latimer, a Pentagon speechwriter under Gates' predecessor, Donald H. Rumsfeld, compares Gates to the Harvey Keitel character in “;Pulp Fiction”; — the one who shows up after the grisly murder to wipe away all traces of blood.

Now that Gates has evolved from the clean-up guy to one of the most powerful members of the Obama cabinet, senior officials at the Pentagon have come up with their own nickname for him: “;The Godfather.”;

The missile defense decision demonstrated both the awkwardness and potency of Gates' position. The Obama team arrived in office skeptical of the plan Gates had signed off on in December 2006 to build a system in Eastern Europe to counter potential Iranian intercontinental ballistic missiles.

A new intelligence estimate on global ballistic missile threats in May concluded that Iran was making less progress than expected on such long-range missiles, but rapidly building short- and medium-range missiles that would not be stopped by the Bush program. Gates accepted that the threat had probably shifted, officials said, and that changing technology meant that the United States could counter shorter-range missiles more effectively with an expanded ship-based SM-3 system.

But officials debated whether to also continue the Bush program. Gates wanted to keep going in case Iran made a breakthrough in longer-range missiles; other officials wanted a clean break from the old system. In the end, at Gates' insistence, the government will continue to finance research and development on ground-based interceptors that were at the heart of the Bush plan while deploying the new system.

“;Secretary Gates played a pivotal role,”; said James L. Jones, the national security adviser. “;It was a rich and robust discussion. If there was a dramatic moment, it was when Secretary Gates affirmatively and without hesitation said this is a better solution.”;

On Afghanistan, Gates has repeatedly declared his concern that more troops would make Americans look increasingly like occupiers. But he has recently softened that opposition, citing McChrystal's argument that an occupation is defined less by numbers than by how troops carry out their mission.

Whatever the president decides in coming weeks, it will fall again to Gates to sell it — to the armed forces, to Congress and to the public. “;We need to understand that the decisions that the president faces on Afghanistan are some of the most important he may face in his presidency,”; Gates said at the Pentagon last week. “;Frankly from my standpoint, everybody ought to take a deep breath.”;