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WASHINGTON - It's a decision that is clearly the most difficult of Barack Obama's young presidency -- whether to heed the pleas of top military officials to send more troops to Afghanistan in a conflict some fear could become his Vietnam.

The president's dilemma has drawn parallels to Lyndon Johnson's deliberations about Vietnam 45 years ago as Obama grows noticeably thinner and confesses to skipping meals as he ponders the risks of escalating the U.S. presence in Afghanistan.

Like Johnson, Obama came to power with an ambitious domestic agenda as a controversial war raged overseas. His presidency hasn't yet been hijacked by an enormous U.S. casualty rate in a faraway land against a stealthy enemy, but his closest advisers worry that it could be.

"The lesson of Vietnam surely is how can you get a nation engaged in it? It seems hard to imagine that Afghanistan is ever going to be a popular war," said Stephen Hess, who worked for Richard Nixon as the Republican president dealt with "Johnson's war" after his 1968 election.

"The public is already against it, and if you're bucking that, you have to be sure that you're awfully determined. You can't have any reservations. You have to be incredibly thick-skinned to be willing to send U.S. troops to their possible deaths, so we're going to learn a lot about him both as a man and as a president throughout this process."

In a book on Johnson, "Flawed Giant," the Texan president confides in an aide about his own time spent agonizing over whether to send U.S. ground troops to Vietnam and escalate the conflict started by his late predecessor, John F. Kennedy.

"The more I stayed awake last night thinking about this thing, the more a it looks like to me we're getting in to another Korea," the book's author, Robert Dallek, quotes Johnson as saying in May of 1964.

"And I don't think it's worth fighting for and I don't think we can get out."

Of course, Johnson ended up sending combat units, and the conflict defined the following four years of his presidency. Almost 60,000 U.S. soldiers died in a war that still stands out as a searingly painful chapter in American history.

Obama is not blind to the similarities, nor are his closest confidantes. His vice-president, Joe Biden, and his chief of staff, Rahm Emanuel, are reportedly opposed to a troop buildup, at odds with the top military commanders pushing the president for more soldiers.

As Obama mulls over the correct course, he's reportedly ordered his closest advisers to look back in history -- particularly the war in Vietnam.

Historian Gordon Goldstein's book on the conflict, "Lessons in Disaster: McGeorge Bundy and the Path to War in Vietnam," is on the president's required reading list. The book chronicles how Bundy, one of the key architects of Vietnam, came to regret America's involvement in the conflict.

Obama is clearly keen to avoid Johnson's fate, but he's also reportedly pointed out to those in his inner circle some crucial differences between the two conflicts: chiefly, that the Vietnamese communists wanted to unite their country, while al Qaida's primary aim is America's destruction.

North Korea, as well, served little strategic purpose to the United States except as a chess piece in its Cold War maneuverings.

"Our quest in Afghanistan, on the other hand, is entwined with the interests of our ally, Pakistan, a nuclear nation and a crucial security interest of the United States," Goldstein wrote in the Los Angeles Times on Thursday.

And yet, there are striking similarities, Goldstein added.

In 1961, Goldstein wrote, Maj. Gen. Edward Lansdale reported to Kennedy that "Vietnam is in critical condition . . . requiring emergency treatment." Lansdale warned the president that without drastic action, the government would be overthrown in in months.

Kennedy dismissed the assessment. Incidentally, Lansdale later became the subject of a Kennedy assassination conspiracy theory when the longtime CIA operative was allegedly spotted at Dealey Plaza in Dallas on the day Kennedy was slain.

Like Lansdale, Gen. Stanley McChrystal has also predicted that without more troops and resources, the war in Afghanistan "will likely result in failure" within a year, Goldstein pointed out.

"Although Vietnam and Afghanistan are disparate wars separated by decades and different national interests, the core questions the commander-in-chief must resolve remain remarkably similar," Goldstein wrote.

Hess points out the unenviable positions both Nixon and Obama were placed in as reluctant wartime presidents.

"Once you're in them, these are exceedingly difficult wars to get out of, and ego tends to get involved," said Hess, now a senior fellow at the Brookings Institution think tank in Washington.

"Nixon campaigned on his intention to get out of Vietnam, but he also didn't want to be known as the first president to lose a war. You can be sure those sorts of thoughts are also occurring right now to people in the White House."

And yet Obama would apparently have the support of many Americans if he opted against sending more troops to Afghanistan. A new Gallup poll suggests 51 per cent of Americans oppose sending new troops, including 44 per cent who said it's time to start winding down the U.S. involvement in Afghanistan.

Earlier this week, Obama rejected the options that his so-called war council presented to him and has asked for revisions over the next few weeks. The news comes in the wake of a warning by the U.S. envoy to Afghanistan, Karl Eikenberry, a former commander in the country.

Eikenberry urged Obama against a buildup until the country's president, Hamid Karzai, proves he's serious about tackling the government corruption that has fuelled the rise of the Taliban.

More U.S. troops, Eikenberry has cautioned Obama, will simply serve to prop up a weak and corruption-plagued government. Karzai reportedly contacted the ambassador's office on Thursday in response to his warning.

At the war council meeting, Obama also reportedly wanted an exit strategy -- a road map detailing how and when U.S. troops would turn over responsibility to the Afghan government.

He's considering options that include adding 30,000 or more U.S. forces to take on the Taliban in key areas of Afghanistan and to buy time for the Afghan government's inadequate and ill-equipped fighting forces to prepare to take over.