WASHINGTON -- The demise of Osama bin Laden complicates what was already a difficult call for President Barack Obama -- how to wind down the nearly decade-old war in Afghanistan now that the symbolic reason for staying in the fight -- to get al-Qaida's leader and avenge 9/11 -- has been undercut.

Momentum had been building in Congress and elsewhere for a shift to a narrower, less costly military mission in Afghanistan even before the U.S. raid that killed bin Laden.

This could suit Obama's desire to put Afghanistan behind him by beginning a phased troop pullout this summer, along with NATO partners.

But it also could put him at sharper odds with his military commanders, who argue for a slower drawdown and a longer-term military commitment that they believe would lessen the chances of Afghanistan again falling apart.

U.S. commanders fear squandering hard-fought battlefield gains, particularly those achieved with the addition last year of an extra 30,000 American troops. They now face a spring offensive by the Taliban, whose goal remains undermining the Afghan government, discrediting its security forces and driving out U.S. troops.

Sen. Dick Durbin, D-Ill., reflected a wider skepticism about remaining heavily involved in Afghanistan when he said Tuesday that he had not imagined at the outset of the war in October 2001 that U.S. troops would still be there "with no end in sight, even after the death of Osama bin Laden."

Top administration officials have vowed not to abandon Afghanistan, even as the U.S. military role shrinks, and their central rationale is not changed by the elimination of bin Laden.

They point to 1989 and the U.S. decision to walk away from Afghanistan after the Soviet occupation collapsed; chaos ensued, the Taliban rose to power and al-Qaida had a launch pad for global terror.

The worry is that the pattern would be repeated if the U.S. left soon, giving terrorists a haven and compelling a future president to intervene again.

"Nobody wants (U.S. troops) to leave and come home more than I do, but I don't want them to go back," said South Carolina Sen. Lindsey Graham, an outspoken supporter of Obama's plan to keep troops there until at least 2014. His advice to the president: "Stay with the plan you got."

House Speaker John Boehner, R-Ohio, who traveled to Afghanistan, Iraq and Pakistan last month, said it was clear that the administration's plan is working. "I don't want to see us take any steps that jeopardize the progress we've made," Boehner said.

Coincidentally, just one day before Obama gave the go-ahead for the raid on bin Laden's compound in Pakistan, he announced a new U.S. lineup in Afghanistan, with Lt. Gen. John Allen replacing Gen. David Petraeus as the military commander and Ryan Crocker succeeding Karl Eikenberry as the top U.S. diplomat.

Sen. John Kerry, D-Mass., acknowledged the likelihood that bin Laden's death would increase public pressure to bow out.

"Some people will ask why we don't pack up and leave Afghanistan. We can't do that," said Kerry, who will travel to Afghanistan in the coming weeks. "But it is no longer enough to simply lay out our goals. We need to determine what type of Afghanistan we plan to leave in our wake so that we may actually achieve these objectives."

Bin Laden's death also highlighted rising U.S. frustration with Pakistan, whose fragile government is heavily funded by Washington as a hedge against militants. Pakistan has a history of supporting some militant groups that target outsiders, including U.S. soldiers in Afghanistan.

White House spokesman Jay Carney said the U.S. is committed to cooperating with Pakistan despite questions about who in the Islamabad government may have known that bin Laden was in hiding in Abbottabad.

"We are working very hard on that relationship. It is an important and complicated relationship that has been tested in many ways over the years," Carney said. "We don't know who if anybody in the government was aware that bin Laden or a high value target was living in the compound. It's logical to assume he had a supporting network. What constituted that network remains to be seen."