Final answer:
The Mongols began conquering new lands in search of wealth and new trade and raiding opportunities, and to distribute the spoils of war as part of their empire's strategy which necessitated constant expansion.
Step-by-step explanation:
According to the expert from "The Mongols in World History," two reasons the Mongols began conquering new lands were the pursuit of wealth and the necessity of new trading and raiding opportunities. The vast amount of wealth seized from the Jin changed the Mongol leadership's mindset, leading to a demand for more goods, food, grazing lands, and raw materials. This demand prompted Chinggis Khan to explore farther west for trading and raiding opportunities. Additionally, the Mongol Empire's expansion provided them direct borders with other realms, such as the Islamic world through Khwarazmia, prompting further conquests to establish trade relations and secure valuable resources.
The Mongols' extraordinary military success and expansionism also meant that they could distribute the spoils of war, which was an essential part of their strategy and a motivation for their constant push into new territories. However, once they reached their westernmost frontiers after battles like Ain Jalut, the expansion stopped, and this posed a problem as their success hin_ed on continuous conquest and the acquisition of new wealth.