Final answer:
Genghis Khan transformed the Mongol Empire into the most formidable fighting force of the era, with their success stemming primarily from their skilled cavalry and innovative tactics.
Step-by-step explanation:
Genghis Khan developed the Mongol Empire into the most skilled fighters in the world of that time. The key to the military success of the Mongols was their extraordinary army, mainly comprised of cavalry. This efficient force was well-organized and disciplined, mastering both archery and horsemanship. Their innovative battlefield tactics, which included feigned retreats and nocturnal assaults, required a high level of coordination and were honed during large hunting expeditions known as nerge. These tactics, combined with a fast, mobile cavalry, made the Mongol soldiers fearsomely effective in siege warfare as they expanded the Mongol Empire to become the largest contiguous empire in history.