Saylor.org's Early Globalizations: East Meets West (1200s-1600s)/Reasons for Conquest
What led to the explosion of the Mongol conquests in the 13th century that created the largest contiguous land empire in world history? On the one hand, between 1180 and 1220, average temperatures dropped in Mongolia, meaning that grass could no longer grow as quickly or as plentifully as it had before. This destruction of grass would prove fatal for the Mongol way of life, as their nomadic lifestyle was so dependent on their animals. If their animals were unable to eat grass, this would have been a strong impetus to move beyond Mongolia. Another reason is perhaps due to the attempt by Northern China to reduce trade with the Mongols. If the Mongols could no longer receive the grain and manufactured goods, at least in the same quantities they had before, they would be prompted to attack and raid, particularly against the Jin dynasty that ruled China. One other popular theory used to explain the Mongol conquests is that Chinggis Khan believed he was charged by Tenggeri, the Mongolian Sky God, to bring all the world under the Mongolian faith, by sword.