Ever hear of the Swamp Rats in the Revolutionary War? The were a small contingent but very effective because they killed the officers first and then hid. The British went ballitic over those tactics because it wasn't with the accepted rules of engagement-two armies facing each other on a large space. I don't want such to happen, but think of all the targets. Like Yamamoto said, "A gun behind every blade of grass."
Better yet, the Afganis chasing out the Russians. Guerilla groups have been chasing out conquerors for centuries.
The Russians left because the Afghans were executing captives right in the field instead of taking prisoners. Probably there were other differences as well along the same lines. The Russian morals of combat were very much like ours at that time and they couldn’t win that way, so they left.