The competing advice and concerns fuel a pivotal struggle to shape the president’s thinking about a war that he inherited but may come to define his tenure. Among the most important outside voices has been that of former Secretary of State Colin L. Powell, a retired four-star Army general, who visited Mr. Obama in the Oval Office this month and expressed skepticism that more troops would guarantee success. According to people briefed on the discussion, Mr. Powell reminded the president of his longstanding view that military missions should be clearly defined.

Mr. Powell is one of the three people outside the administration, along with Senator John F. Kerry and Senator Jack Reed, considered by White House aides to be most influential in this current debate. All have expressed varying degrees of doubt about the wisdom of sending more forces to Afghanistan.

Mr. Kerry, Democrat of Massachusetts and chairman of the Foreign Relations Committee, has warned of repeating the mistakes of Vietnam, where he served, and has floated the idea of a more limited counterterrorist mission. Mr. Reed, Democrat of Rhode Island and an Army veteran, has not ruled out supporting more troops but said “the burden of proof” was on commanders to justify it.

In the West Wing, beyond Vice President Joseph R. Biden Jr., who has advocated an alternative strategy to the troop buildup, other presidential advisers sound dubious about more troops, including Rahm Emanuel, the chief of staff, and Gen. James L. Jones, the national security adviser, according to people who have spoken with them. At the same time, Mr. Obama is also hearing from more hawkish figures, including Secretary of State Hillary Rodham Clinton and Richard Holbrooke, the special representative for Afghanistan and Pakistan.

Nothing shook his faith quite as much as what you might call the Karzai dinners. The first occurred in February 2008, during a fact-finding trip to Afghanistan that Biden took with fellow senators John Kerry and Chuck Hagel. Dining on platters of rice and lamb at the heavily fortified presidential palace in Kabul, Biden and his colleagues grilled Karzai about reports of corruption and the growing opium trade in the country, which the president disingenuously denied. An increasingly impatient Biden challenged Karzai's assertions until he lost his temper. Biden finally stood up and threw down his napkin, declaring, "This meeting is over," before he marched out of the room with Hagel and Kerry. It was a similar story nearly a year later. As Obama prepared to assume the presidency in January, he dispatched Biden on a regional fact-finding trip. Again Biden dined with Karzai, and, again, the meeting was contentious. Reiterating his prior complaints about corruption, Biden warned Karzai that the Bush administration's kid-glove treatment was over; the new team would demand more of him.

Biden's revised view of Karzai was pivotal. Whereas he had once felt that, with sufficient U.S. support, Afghanistan could be stabilized, now he wasn't so sure. "He's aware that a basic rule of counterinsurgency is that you need a reliable local partner," says one person who has worked with Biden in the past. The trip also left Biden wondering about the clarity of America's mission. At the White House, he told colleagues that "if you asked ten different U.S. officials in that country what their mission was, you'd get ten different answers," according to a senior White House aide. He was also growing increasingly concerned about the fate of Pakistan. Biden has been troubled by the overwhelmingly disproportionate allocation of U.S. resources to Afghanistan in comparison to Pakistan, a ratio one administration official measures as 30:1. Indeed, before leaving the Senate last year, Biden authored legislation that would triple U.S. non-military aid to Islamabad to $1.5 billion per year. (House-Senate bickering has tied up the plan for months, and Biden has recently been working the phones to broker a compromise.)

George Stephanopoulos reported that the new “must-read book” for President Obama’s war team is “Lessons in Disaster” by Gordon M. Goldstein, a foreign-policy scholar who had collaborated with McGeorge Bundy, the Kennedy-Johnson national security adviser, on writing a Robert McNamara-style mea culpa about his role as an architect of the Vietnam War.

Bundy left his memoir unfinished at his death in 1996. Goldstein’s book, drawn from Bundy’s ruminations and deep new research, is full of fresh information on how the best and the brightest led America into the fiasco. “Lessons in Disaster” caused only a modest stir when published in November, but The Times Book Review cheered it as “an extraordinary cautionary tale for all Americans.” The reviewer was, of all people, the diplomat Richard Holbrooke, whose career began in Vietnam and who would later be charged with the Afghanistan-Pakistan crisis by the new Obama administration [...]

As Goldstein said to me last week, it’s “eerie” how closely even these political maneuvers track those of a half-century ago, when J.F.K. was weighing whether to send combat troops to Vietnam. Military leaders lobbied for their new mission by planting leaks in the press. Kennedy fired back by authorizing his own leaks, which, like Obama’s, indicated his reservations about whether American combat forces could turn a counterinsurgency strategy into a winnable war.