As poor as Afghanistan can be it has been able to rally within its current borders at least a dozen major ethnic groups - Baluch, Chahar Aimak, Turkmen, Hazara, Pashtun, Tajik, Uzbek, Nuristani, Arab, Kirghiz, Pashai and Persian.
During invasions from many armies during the 1200s -1500s to include Alexander the Great at one time in Afghanistan has failed to conqueror.
I would say that the vast Hindu Kush mountains of Afghanistan are a natural barrier to prevent invasion by surrounding rival armies.
Can you help name a few, who have failed & Why! Please give your Thoughts & Comments on this discussion!