The Mongols succeeded in conquering such a large territory in a short period time with logical tactics and efficient weapons. With the use of fear, structure in the armies, and the weapons used in battle, the Mongols spread their reign throughout Asia, and Eastern Europe in such a short, set period of time. But most of what the Mongols conquered were steppes which weren't very highly populated to start off with. The Chinese or Persians had large populations, but their infantry army was no match for Mongol horse archers. So basically the Mongols were successful because 100k cavalry armies were very formidable.