Here are some of the reasons why the Mongols were so successful as conquerors:
1 they had Genghis Khan - he was a military genius who managed to train his army well and lead them into many battles
2 they were numerous - there were over 10,000 people in the Mongolian army
3 their military strategists were quite crafty - they relied on setting traps and tricking their enemies
4 they used cruelty and fear - everyone was afraid of the Mongols and what they would do to them if they caught them