Vice president Joe Biden says it's up to Afghanistan to take responsibility for its own security.
Republican rival Paul Ryan said he doesn't want the United States to lose the gains achieved in its decade-long war there following the Sept 11, 2001, terrorist attacks.
Biden and Ryan conveyed at Thursday's presidential debate that it's time to wind down US involvement.
Ryan said he agrees with President Barack Obama in transitioning out of the country by 2014, but said the White House should not announce a deadline for withdrawal and expose weakness.
Both men were responding to a question about why American troops shouldn't just leave Afghanistan immediately.