Being able to 'seal' Afg's notoriously leaky borders would be a good start. To do that requires manpower. Cut off the escape routes and means of resupply and you begin to starve the insurgency. That's a good start. Need a heap more troops to do that.
Yes, but the problem with this is that there isn’t a country to be defended. The central government is corrupt and does not control the countryside, so it’s hard to be sure what you would be defending and from whom. Afghanistan is really a loose alignment of tribal and ethnic groups. Moreover, there is an area often called ‘Pashtunistan’ which unites the Pashtun ethnic group across the Durand line. Pashtuns as a rule have more loyalty to each other than to any nation-states called Afghanistan and Pakistan. This is understood by many people on the ground, but I find the implications of this are still not widely realized or understood in Washington.