Extremists are global and their actors are mobile. Thus, a victory in Afghanistan doesn't necessarily mean a victory in the war on terror against al Qaeda.

As the United States moves forward in Afghanistan, some of our reasons for being there in the first place - like al Qaeda - have moved; Osama bin Laden and Ayman al Zawahiri are believed to be hiding in the tribal areas of Pakistan, and the terror group is thought to be discussing a move to Somalia or Yemen.

Extremist groups infest broken, ungoverned places and play on the reigning misery and discontent in order to attract recruits. Unfortunately for the U.S., the Middle East is fraught with broken places.

We can't fix all these places, and Afghanistan is proving that we may not be able to fix any of them. For nation-building and a full-scale counterinsurgency to work in Afghanistan, we likely need to discuss additional forces in terms of hundreds, not tens of thousands, of troops.

With the U.S. enmeshed in Iraq and an economic mess at home, can war-weary Americans and their legislators accept such numbers?

So our focus narrows. We must disabuse ourselves of the notion that we are fighting to win the war on terror.

EACH TIME THE president reassures us that we'll succeed in defeating extremists, he sets himself up for failure.

Extremists will always exist. They usually reside in places like the Mideast, but they can also pop up in places like Oklahoma City and Aurora, Colo. They're born of misery and deficiency, but they're also physicians and sons of billionaires. They live in times of turmoil as well as periods of relative peace. They come from poverty and from privilege.

In the end, our obligation is not to do the impossible, it's to do what we can. What we can do is commit ourselves to engage Afghanistan, alongside a credible Afghani government, for the sake of the Afghani people and for regional stability. The United States needs a renewed sense of purpose in Afghanistan.

There is a reason the United States should remain in Afghanistan. But it's also President Obama's duty to explain clearly to the American people what that reason is. It's not to rid the world of evil or terrorism. Both will always exist.

Patrick Cunnane, of Jenkintown, is a senior at the University of Miami studying political science and journalism.