U.S. troops are still in Afghanistan, nearly 11 years after they invaded. Why? The answer boils down to one word: al-Qaida. The goal is to damage the terrorist group enough to prevent a repeat of the 9/11 attacks.
Where they stand:
After nearly tripling the number of U.S. troops in Afghanistan in 2009-10, President Barack Obama is now pulling them out, aiming to end all U.S. combat there by December 2014. Mitt Romney has outlined a broad goal for the war - without specifics about troop numbers - that is similar to Obama's: Hand over security responsibility to the Afghans at a pace that does not risk the country's collapse and al-Qaida's return.
Why it matters:
Only small numbers of al-Qaida fighters are still in Afghanistan, and their iconic leader, Osama bin Laden, is long dead. But the threat they represent is still the main reason Americans are still fighting and dying there.
The logic goes like this: If U.S. and allied forces were to leave before the Afghans can defend themselves, the Taliban would regain power. And if they were in charge, then al-Qaida would not be far behind.
In that view of what's at stake, al-Qaida would once again have a launching pad for attacks on American soil.
What's often overlooked in that scenario is an answer to this question: Why, after so many years of foreign help, are the Afghans still not capable of self-defense? And who can say when they will get to that point?
The official answer is 2014. By the end of that year, the U.S. and its allies are scheduled to end their combat role. The Afghans will be fully in charge, or so it is hoped, and the war will be over - at least for Americans.
So, from an American point of view, what is at stake in Afghanistan is avoiding a repeat of 9/11. But it is also true that the United States faces threats on other fronts. Some of those threats have arisen as a consequence of the U.S. invasion of Afghanistan in October 2001, just weeks after the traumatizing 9/11 attacks.
Al-Qaida has migrated to other countries such as Pakistan, Yemen, Somalia and various spots in North Africa.
Thus, al-Qaida remains a worry, but its presence in Afghanistan does not seem to trouble many Americans. Although nearly 2,000 U.S. troops have died in Afghanistan, the war is hardly an issue in the presidential campaign.
It's perhaps a measure of the public's inattention to Afghanistan that Defense Secretary Leon Panetta felt it necessary to say at a Pentagon news conference that it was important to "remind the American people that there is a war going on."
He added, with an allusion to the al-Qaida threat: "Young men and women are dying in order to try to protect this country."
The outcome in Afghanistan also is important because of the enormous investment in human lives over the past decade. To let it unravel and revert to a pre-9/11 Taliban rule would be seen by many as dishonoring those sacrifices.