The US won't lose the war if it kills enough people. That means there will be a so-called surge and many, many Afghani civilians will die. The cost of resisting the occupation they hate will become too much to bear any longer. The US will not give way because Afghanistan is a strategic position in the ME that they need. This differs not a bit from the Russian occupation.
This is a war for the exact same reason as the Iraq war. The claims of helping the people or fighting terrorism or the latest, preventing a ground for terrorism are hollow claims and complete fabrications and lies.
The US will not leave either Afghanistan or Iraq and huge US bases are being built and have been built in both countries for a permanent occupation. The US doesn't even really deny this fact.
But as we have witnessed in Iraq, the few citizens who resist will fight on and kill the military occupiers of their country. As they will in Afghanistan and as they proved they were capable of doing against the Russians. The occupation's military will never walk safely on the streets of either country because there will always be a few brave souls left who will try to kill them or drive them from their country. Brave souls who fight on at great personal risk to themselves.