It was just one brief exchange about Afghanistan with an aide late in 2009, but it suggests how President Obama’s thinking about what he once called “a war of necessity” began to radically change less than a year after he took up residency in the White House.

Not long before, after a highly contentious debate within a war cabinet that was riddled with leaks, Mr. Obama had reluctantly decided to order a surge of more than 30,000 troops. The aide told Mr. Obama that he believed military leaders had agreed to the tight schedule to begin withdrawing those troops just 18 months later only because they thought they could persuade an inexperienced president to grant more time if they demanded it.

“Well,” Mr. Obama responded that day, “I’m not going to give them more time.”

A year later, when the president and a half-dozen White House aides began to plan for the withdrawal, the generals were cut out entirely. There was no debate, and there were no leaks. And when Mr. Obama joins the leaders of other NATO nations in Chicago on Sunday and Monday, the full extent of how his thinking on Afghanistan has changed will be apparent. He will announce what he has already told the leaders in private: All combat operations led by American forces will cease in summer 2013, when the United States and other NATO forces move to a “support role” whether the Afghan military can secure the country or not.

Mr. Obama concluded in his first year that the Bush-era dream of remaking Afghanistan was a fantasy, and that the far greater threat to the United States was an unstable, nuclear-armed Pakistan. So he narrowed the goals in Afghanistan, and narrowed them again, until he could make the case that America had achieved limited objectives in a war that was, in any traditional sense, unwinnable.

“Just think how big a reversal of approach this was in just two years,” one official involved in the administration debates on Afghanistan said. “We started with what everyone thought was a pragmatic vision but, at its core, was a plan for changing the way Afghanistan is wired. We ended up thinking about how to do as little wiring as possible.”

The lessons Mr. Obama has learned in Afghanistan have been crucial to shaping his presidency. Fatigue and frustration with the war have defined the strategies his administration has adopted to guide how America intervenes in the world’s messiest conflicts. Out of the experience emerged Mr. Obama’s “light footprint” strategy, in which the United States strikes from a distance but does not engage in years-long, enervating occupations. That doctrine shaped the president’s thinking about how to deal with the challenges that followed — Libya, Syria and a nuclear Iran.

In interviews over the past 18 months, Mr. Obama’s top national security aides described the evolution of the president’s views on Afghanistan as a result of three rude discoveries.

Mr. Obama began to question why Americans were dying to prop up a leader, President Hamid Karzai of Afghanistan, who was volatile, unreliable and willing to manipulate the ballot box. Faced with an economic crisis at home and a fiscal crisis that Mr. Obama knew would eventually require deep limits on Pentagon spending, he was also shocked, they said, by what the war’s cost would be if the generals’ counterinsurgency plan were left on autopilot — $1 trillion over 10 years. And the more he delved into what it would take to truly change Afghan society, the more he concluded that the task was so overwhelming that it would make little difference whether a large American and NATO force remained for 2 more years, 5 more years or 10 more years.

The remaking of American strategy in Afghanistan began, though no one knew it at the time, in a cramped conference room in Mr. Obama’s transition headquarters in late 2008. Gen. Douglas E. Lute, who had spent the last two years of the Bush administration trying to manage the many trade-offs necessary as the Iraq war consumed troop and intelligence resources needed in Afghanistan, arrived with a PowerPoint presentation.

The first slide that General Lute threw onto the screen caught the eye of Thomas E. Donilon, later President Obama’s national security adviser. “It said we do not have a strategy in Afghanistan that you can articulate or achieve,” Mr. Donilon recalled three years later. “We had been at war for eight years, and no one could explain the strategy.”

So in the first days of his presidency, Mr. Obama asked Bruce O. Riedel, a former C.I.A. officer with deep knowledge of the region, to lead a rapid review. At the time, the president was still speaking in campaign mode. He talked about remaking “an economy that isn’t dominated by illicit drugs” in Afghanistan and a “civilian surge” to match the military effort. But he said little about the Riedel team’s central insight: that Pakistan posed a far greater threat.

“If we were honest with ourselves, we would call this problem ‘Pak/Af,’ not ‘Af/Pak,’ ” Mr. Riedel said shortly after turning in his report. But the White House would not dare admit that publicly — even that rhetorical reversal would further alienate the Pakistanis.

Mr. Obama agreed with Mr. Riedel, but thought the review did not point clearly enough toward a new strategy. To get it right, the president ordered up a far more thorough process that would involve everyone — military commanders and experts on civilian reconstruction, diplomats who could explore a negotiation with the Taliban, and intelligence officials who could assess which side of the war the Pakistanis were fighting on.

But he also began to reassess whether emerging victorious in Afghanistan was as necessary as he had once proclaimed. Ultimately, Mr. Obama agreed to double the size of the American force while training the Afghan armed forces, but famously insisted that, whether America was winning or losing, the drawdown would begin in just 18 months.

“I think he hated the idea from the beginning,” one of his advisers said of the surge. “He understood why we needed to try, to knock back the Taliban. But the military was ‘all in,’ as they say, and Obama wasn’t.”

The president’s doubts were cemented as the early efforts to take towns like Marja in Helmand Province took months longer than expected. To Mr. Obama and his aides, Marja proved that progress was possible — but not on the kind of timeline that Mr. Obama thought economically or politically affordable.

“Marja looks a lot better than two years ago,” one senior official said at the end of last year. “But how many Marjas do we need to do, and over what time frame?”

The tight group of presidential aides charged with answering questions like that — of redefining the mission — began meeting on weekends at the end of 2010. The group’s informal name said it all: “Afghan Good Enough.”

“We spent the time asking questions like: How much corruption can we live with?” one participant recalled. “Is there another way — a way the Pentagon might not be telling us about — to speed the withdrawal? What’s the least we can spend on training Afghan troops and still get a credible result?”

By early 2011, Mr. Obama had seen enough. He told his staff to arrange a speedy, orderly exit from Afghanistan. This time there would be no announced national security meetings, no debates with the generals. Even Defense Secretary Robert M. Gates and Secretary of State Hillary Rodham Clinton were left out until the final six weeks.

The key decisions had essentially been made already when Gen. David H. Petraeus, in his last months as commander in Afghanistan, arrived in Washington with a set of options for the president that called for a slow withdrawal of surge troops. He wanted to keep as many troops as possible in Afghanistan through the next fighting season, with a steep drop to follow. Mr. Obama concluded that the Pentagon had not internalized that the goal was not to defeat the Taliban. He said he “believed that we had a more limited set of objectives that could be accomplished by bringing the military out at a faster clip,” an aide reported.

After a short internal debate, Mr. Gates and Mrs. Clinton came up with a different option: end the surge by September 2012 — after the summer fighting season, but before the election. Mr. Obama concurred. But he was placing an enormous bet: his goals now focus largely on finishing off Al Qaeda and keeping Pakistan’s nuclear weapons from going astray. Left unclear is how America will respond if a Taliban resurgence takes over wide swathes of the country America invaded in 2001 and plans to largely depart 13 years later.

This article is adapted from “Confront and Conceal: Obama’s Secret Wars and Surprising Use of American Power,” to be published by Crown on June 5. (Copyright 2012 The New York Times Company 05/20/12)