[ohioman]

In my book, good ‘ole Genghis and family were the #1 butt-kickers of all time. They were brutal (which was normal for the 13th century); however, they were undefeated and had an empire for almost 200 years. It has always amazed me that a power like this could emerge from such a sparsely populated area. As a result, War colleges across the world have studied their tactics for hundreds of years. The modern tendency to dismiss them as simple savages is misguided. Genghis was absolutely brilliant.

[Bubba Ho-Tep]

I don't know if you're a podcast guy, and it's behind the paywall now so you have to pay $1.99 an episode, but Dan Carlin's Hardcore History did a terrific series of five episodes on the Mongols--8 hours worth. Wrath of the Khans

[NorseViking]

The key to their success was a numerous organized military while most of their opponents were early feudal meaning a sort of mafia with king as a godfather, barons for bosses and knights for the mobsters. Early feudals were good to run a protection racket against tradesmen and peasants or to protect their turf against other such mobs read kingdoms but they stood no chance against Mongols fielding organized armies or highly trained horsemen numbered in tens of thousand.