Can a president declare a war must be won but not proclaim he'll wage that war for as many years as is necessary to succeed? That is the contradiction that President Obama will be obligated to explain on Tuesday night when he addresses the nation from West Point to explain his apparent decision to send some 30,000 additional U.S. soldiers to the cauldron of Afghanistan.

Obama is in a tight corner. He is entering battle without the full support of his natural allies. Democratic voters and legislators are skeptical of expanding the war; Republicans favor it. So those who generally want the Obama presidency to succeed are about to be alienated big-time; simultaneously, Obama will find most of his political support for the troops increase among those who usually yearn for his failure. That's a prescription for political trouble.

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More important, though, is Obama's effort to preserve his credibility as commander in chief. This is a tougher task than navigating the politics. Over the past few months, Obama and his aides have repeatedly vowed that their commitment in Afghanistan is not "open-ended." Yet just a few days ago, as Obama telegraphed his decision to dispatch more troops, the president made a firm promise to "finish the job" in Afghanistan. That sounded like an open-ended commitment, for who knows how long it will take to accomplish the mission in Afghanistan?

There is the complicated matter of defining that mission. Is it to eradicate the Taliban -- literally? Destroy all al-Qaeda remnants? Stand up an Afghan government and military that can function and keep the Taliban and al-Qaeda at bay permanently? But whatever the finer points of his Afghanistan to-do list, Obama is conveying a dual message at war with itself: This must be done; this won't last forever.

If the job in Afghanistan is critical to the existence of the United States, why not fight it forever? And if it's not absolutely essential to the survival of the United States, why fight it at all? Obama is threading the needle, presenting the war as necessary but assuring the public it won't be a long-term endeavor. (Can you say "quagmire"?) As he takes full possession of the war that George W. Bush and Dick Cheney screwed up, Obama might for a while be able to get away with this double-edged rhetoric. But he is putting down a marker that the public won't forget -- especially when 2012 rolls around.

It's true that Obama is likely to be judged mostly by the unemployment numbers, but he is also betting his presidency on Afghanistan. And that means he will have a lot of explaining to do, as he expands the war. For instance, this White House claims that success is only possible if there is a credible partner in Kabul. Yet the government of Hamid Karzai is corrupt and inept. Thus, what is to be done? If Obama fully acknowledges the mess in Kabul, he undermines his war. If he does not, he can be accused of defying (or denying) reality. Handling this thorny and fundamental issue is another challenge for the White House team working on Obama's West Point speech.

Ultimately, Obama has to defend an expansion of a war supported by less than half of the public, address the serious problems at hand (some of which have no obvious solution), provide a vision of success, and convince Americans this is not a never-ending slog that will break what's left of the bank. (By the way, it's possible, as terrorism expert Peter Bergen has testified that al-Qaeda -- the raison d'etre of the Afghanistan war -- is no longer a "direct national security threat to the United States.") Afghanistan presents a series of profound policy dilemmas. Selling such a war is a dilemma as well.

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