This dissertation is an analysis of how Europe responded to the Mongols in the thirteenth- and fourteenth-centuries. In only sixty years the Mongols created the largest land empire the world has ever seen. After Mongol attacks on Eastern Europe---specifically Russia, Poland, Hungary, and the Balkans---in the 1230s--1240s, Europeans were petrified of the terrifying and unfamiliar invaders. However, at times this initial anxiety waned and even turned to hope for a European alliance with the Mongols against Muslim powers like the Mamluks of Egypt. Using travel narratives of missionaries who traveled in Mongol territories, diplomatic correspondence between khans, popes, and kings, evidence of panni tartarici (Tartar silk) in Europe, theological treatises, and tombstones of Europeans who died in Yuan China, I challenge a secondary literature that tends to portray European-Mongol relations during this period as static. I show how European understandings of the Mongols varied greatly and depended on contemporary political events. I argue that medieval Europeans saw the Mongols as both threat and ally, as a sign of the Apocalypse and a partner in the fight against Islam. These sentiments were not mutually exclusive. This study contributes to studies of world history and understandings of pre-modern transnational relations. Its diverse sources illustrate the complexity of European attitudes towards the Mongols, and demonstrate how important an interdisciplinary approach is for the study of the Mongol Empire.