General Stanley McChrystal confirmed in Congress that President Obama's review of the strategy had allowed for 'greater clarity' to the mission.
Although General McChrystal warned that the Afghan mission was 'undeniably difficult,' he was confident that the insurgency would lose momentum.
30,000 extra troops are being deployed to Afghanistan.
Mc Chrystal told Congress that killing or capturing Osama bin Laden is the key to defeating the al-Qaeda terrorist network.
Senior US diplomat Richard Holbrooke acknowledged today that institution-building in Afghanistan would have to start 'from scratch' in the ninth year of engagement.
He admitted the country had 'enormous' problems including corruption, a booming drug trade and high illiteracy, even among civil servants.
Mr Holbrooke said he had ordered reading and writing to be included in training for security forces.
He said it would be 'nice' if the goal of having 160,000 trained Afghan police officers in three to four years were achieved.
Mr Holbrooke said the biggest civilian aid programme was for the agricultural sector, which until the late 1970s had allowed Afghanistan to export goods throughout Central Asia.
Asked whether US allies should send more troops or more agricultural consultants to the country, Mr Holbrooke replied: 'Both would be good.'
