Well, The Mongols were really mean and conquered areas easily since everyone was affraid of them. Genghis Khan the leader of the Mongols and the Mongols themselves would come in and invade an area, killing anyone who disagreed. Eventually like most armies they weaked over time. I believe it was a guy named (Firstnameidkit) Kublai who took control over the Mognols last, with his death the Mongol Empire just kind of fell apart.