Why marching through Afghanistan does not secure it

Afghanistan's geography allows an army to enter while making sustained control difficult. British weapons appear among opponents of Russian intervention, and Russian weapons among opponents of Britain, although captured equipment can also explain their presence. Mountain passes channel movement, while winter can close routes and isolate forces. Armies that win pitched battles can still lose campaigns of attrition and guerrilla warfare. Holding the ground beyond successive ridges requires dispersal into small detachments because troops cannot observe what happens beyond each mountain. That loss of concentration leaves parts of a large army vulnerable to defeat separately.