It is clear from President Barack Obama's changing position on Afghanistan that the war's frustrating complexity is finally sinking in. As a candidate, Obama was unequivocal about ramping up there because, he said, it was "the war we have to win" and, unlike Iraq, the place where America's military efforts needed to be focused. In February, he ordered 21,000 troops to Afghanistan to supplement the 38,000 already there at the time.

Now, however, Obama wants a rethink and is even open to considering a troop reduction, despite his Afghanistan commander's urgent call for more troops. The scant mention of Afghanistan in Obama's U.N. speech this week suggested it is dropping on the president's list of international priorities.

America cannot afford to get it wrong in that corner of the world. Whatever Obama decides, he must be resolute publicly, even if he has deep personal doubts. Waffling hurts his cause and damages his credibility.

Afghanistan offers no easy choices. Whatever the course, the path forward will be fraught with peril. First and foremost, the president must decide whom we should be fighting. America entered Afghanistan after the 9/11 attacks specifically to root out al-Qaeda. Pursuing al-Qaeda's hosts, the Taliban, was a secondary objective. Remember that the Taliban was in power well before 9/11, and its leaders were tolerated if not courted by the United States for years.

The easy choice would be to reduce the mission's scope and refocus on al-Qaeda. It would be crisp, clean and would require far fewer troops. Reducing America's footprint would undoubtedly help cut U.S. troop casualties significantly. There is a huge drawback, however: Lacking a major military effort to hold Taliban forces at bay, their strength and influence would only grow — not just in Afghanistan but in neighboring Pakistan as well. Reducing the footprint doesn't necessarily reduce our headaches.

Going the opposite direction — raising troop levels to fight the Taliban — would add to our troops' exposure and casualties. It would mean more bases and raise the U.S. profile in Afghan eyes as an occupier. This route entails a deeper long-term commitment and all the nation-building burdens that accompany it — government corruption, reconstruction and rampant opium trafficking, to name a few. But it also would mean denying the Taliban and its al-Qaeda allies the power and resources they seek.

It's not a simple choice between committing more deeply or withdrawing altogether. The United States is in Afghanistan for the long haul no matter what, because there is no viable Afghan or international security force to assume the U.S. military role. The president's only realistic choices are to define the enemy and determine the military commitment necessary to defeat that enemy.

In spite of all that has divided our country, Afghanistan used to be the one thing that Americans rallied around as the good fight for the right reason. As polls suggest, though, Americans won't support a war against an ill-defined enemy, with an amorphous goal, run by an indecisive president.