Military power is one of the most coveted forms of power. It affords a leader stability and the ability to crush their enemies.
Genghis Kahn is a candidate for the most powerful leaders of all time because of the military power he exercised relative to other powers at the time. The Mongol Empire had a formidable army unmatched by any other military at the time. The only other empire that even came close to the level of military dominance exerted by the Mongol Empire would be the British Empire. However, no individual in the British Empire enjoyed a level of power comparable to Genghis Kahn. The British monarch was limited by Parliament, and Prime Ministers were not autocratic either.
Military power is not as powerful as ideological power. This can be seen through history. The Soviet Union's defeat in Afghanistan and the US defeat in Vietnam are two clear indicators that military power has its limits. To become truly powerful, a leader must capture a population's hearts and minds. Genghis Kahn failed to capture people's hearts and minds. He forced his leadership on people and built an army of slaves. If he had been around today, an insurgency movement would have plagued his leadership and he would have been unable to build an empire.
Military power is a valuable source of power. Genghis Kahn amassed an unstoppable and unprecedented amount of military power.
[P1] The most powerful leader in history had a military that enjoyed relative dominance over other military forces. [P2] Genghis Kahn's Mongols were more dominant than any other military force in history. [P3] Therefore, Genghis Kahn was the most powerful leader in history.
Rejecting the premises
[Rejecting P1] Military power is far less important than ideological power. [Rejecting P3] Genghis Kahn amassed almost no ideological power.