Thank you for the opportunity to help you with your question!
When Genghis Khan, first known as Temujin, was a kid, there were many different Mongolian tribes. The tribes often fought with each other, and i think that Temujin wanted to unite his people. He used a powerful military, scare tactics, and set up "spies" in some tribes. After he united all the tribes, I think he got the name geneghis khan.
i think once he conquered the Han dynasty early in his career he though he could take on anyone and win, so he set out to do just that and he kept on winning, eventually having the biggest continuous land empire in the history of the world.
Please let me know if you need any clarification. I'm always happy to answer your questions.