WASHINGTON—On the afternoon he held the eighth meeting of his Afghanistan review, President Barack Obama arrived in the White House Situation Room ruminating about war. He had come from Arlington National Cemetery, where he had wandered among the chalky white tombstones of those who had fallen in the rugged mountains of Central Asia.

How much their sacrifice weighed on him that Veterans Day last month, he did not say. But his advisers say he was haunted by the human toll as he wrestled with what to do about the eight-year-old war. Just a month earlier, he had mentioned to them his visits to wounded soldiers at the Army hospital in Washington. “;I don't want to be going to Walter Reed for another eight years,”; he said then.

The economic cost was troubling him as well after he received a private budget memo estimating that an expanded presence would cost $1 trillion over 10 years, roughly the same as his health care plan. Now as his top military adviser ran through a slide show of options, Obama expressed frustration. He held up a chart showing how reinforcements would flow into Afghanistan over 18 months and eventually begin to pull out, a bell curve that meant American forces would be there for years to come.

“;I want this pushed to the left,”; he told advisers, pointing to the bell curve. In other words, the troops should be in sooner, then out sooner.

When the history of the Obama presidency is written, that day with the chart may prove to be a turning point, the moment a young commander in chief set in motion a high-stakes gamble to turn around a losing war. By moving the bell curve to the left, Obama decided to send 30,000 troops mostly in the next six months and then begin pulling them out a year after that, betting that a quick jolt of extra forces could knock the enemy back on its heels enough for the Afghans to take over the fight.

The three-month review that led to the escalate-then-exit strategy is a case study in decision-making in the Obama White House—intense, methodical, rigorous, earnest and at times deeply frustrating for nearly all involved. It was a virtual seminar in Afghanistan and Pakistan, led by a president described by one participant as something “;between a college professor and a gentle cross-examiner.”;

Obama peppered advisers with questions and showed an insatiable demand for information, taxing analysts who prepared three dozen intelligence reports for him and Pentagon staff members who churned out thousands of pages of documents.

This account of how the president reached his decision is based on dozens of interviews with participants as well as a review of notes some of them took during Obama's 10 meetings with his national security team. Most of those interviewed spoke on the condition of anonymity to discuss internal deliberations, but their accounts have been matched against those of other participants wherever possible.

Obama devoted so much time to the Afghan issue—nearly 11 hours on the day after Thanksgiving alone—that he joked, “;I've got more deeply in the weeds than a president should, and now you guys need to solve this.”; He invited competing voices to debate in front of him, while guarding his own thoughts. Even David Axelrod, arguably his closest adviser, did not know where Obama would come out until just before Thanksgiving.

With the result uncertain, the outsize personalities on his team vied for his favor, sometimes sharply disagreeing as they made their arguments. The White House suspected the military of leaking details of the review to put pressure on the president. The military and the State Department suspected the White House of leaking to undercut the case for more troops. The president erupted at the leaks with an anger advisers had rarely seen, but did little to shut down the public clash within his own government.

“;The president welcomed a full range of opinions and invited contrary points of view,”; Secretary of State Hillary Rodham Clinton said in an interview last month. “;And I thought it was a very healthy experience because people took him up on it. And one thing we didn't want—to have a decision made and then have somebody say, 'Oh, by the way.' No, come forward now or forever hold your peace.”;

The decision represents a complicated evolution in Obama's thinking. He began the process clearly skeptical of Gen. Stanley A. McChrystal's request for 40,000 more troops, but the more he learned about the consequences of failure, and the more he narrowed the mission, the more he gravitated toward a robust if temporary buildup, guided in particular by Defense Secretary Robert M. Gates.

Yet even now, he appears ambivalent about what some call “;Obama's war.”; Just two weeks before McChrystal warned of failure at the end of August, Obama described Afghanistan as a “;war of necessity.”; When he announced his new strategy last week, those words were nowhere to be found. Instead, while recommitting to the war on al-Qaida, he made clear that the larger struggle for Afghanistan had to be balanced against the cost in blood and treasure and brought to an end.

Aides, though, said the arduous review gave Obama comfort that he had found the best course he could. “;The process was exhaustive, but any time you get the president of the United States to devote 25 hours, anytime you get that kind of commitment, you know it was serious business,”; said Gen. James L. Jones, the president's national security adviser. “;From the very first meeting, everyone started with set opinions. And no opinion was the same by the end of the process.”;