Genghis Khan used terror as a strategic weapon. If a city surrendered, it was spared. If it resisted, every living thing was slaughtered. This reputation traveled ahead of him, causing entire nations to submit without a fight.
This was not mindless cruelty; it was cold calculation. By being utterly ruthless to those who resisted, he minimized future resistance. He valued human life only as a resource or an obstacle.
His most famous tactic. The Mongols would flee, seemingly in panic. The enemy, sensing an easy victory, would break formation to chase them. Suddenly, the Mongols would turn, and the 'fleeing' archers would unleash a storm of death.
Result
The enemy army is drawn out of position and annihilated in the open.