I've always found it, in a very macabre way, rather amusing that a Mongol force of 100,000 invaded and completely destroyed Hungary, and it turns out that this was just an advance scouting army and the conquest was only an afterthought.
Hungary got steamrolled when a Mongol scouting force got bored in Poland
It's a good illustration of how utterly, ridiculously unprepared the armies of the Old World were for the Mongol tactics. They didn't play by their silly rules, they ran circles around their soldiers, they laughed at logistics, and they assimilated everyone else's technology. If a Mongol force of 30,000 could annihilate a Jin army of 500,000, the sad little armies of feudal Europe are gonna get squashed like mosquitoes. The Mongols didn't conquer them because they didn't feel like it.