From 1206 until 1368, much of the Eastern Hemisphere dreaded the invasion and conquests of the Mongols. Their empire, which at its height spanned from Korea to the east and Anatolia and Eastern Europe to the west, was and remains the largest contiguous empire in the history of the world.
Whether the Mongol Empire was good or bad is debatable. Its defenders credit it for reopening the Silk Road, religious tolerance, progressive views on the role of women in society, and its general cosmopolitanism. Its detractors point out its major role in spreading the Bubonic Plague, the Mongols' brutality in war, and widespread destruction of infrastructure in the lands they conquered. Regardless of which side of the debate is correct, there is little question that the Mongols set many of the technological, political, and social progress of the lands and peoples they conquered back significantly.
Prior to the Mongol invasions, China was ruled by the Song Dynasty. During this period of Chinese history, the country experienced new economic innovations that the world would not yet see again for centuries after the Mongol conquests.
These economic innovations were directly the product of what was quite possibly the world's first industrial revolution. During this period, China saw the rise of immense coal-fueled factories, large-scale urbanization, and the production of immense quantities of iron, to name only a few socioeconomic achievements. The conquest of China by the Mongols destroyed these industrial developments, which the world would not see again until England began to industrialize in its own right centuries later.
What the Mongols did to China economically, they did to the Islamic Middle East intellectually. A great deal is known about the great artistic and scientific contributions of the medieval Muslim world, along with the fact that it had been among the most progressive societies the world had seen at that point in history. Mongol destruction of Islamic intellectual innovation is perhaps best epitomized by their destruction of the House of Wisdom in Baghdad in 1258, one of the greatest libraries the world has ever seen. The destruction of the Baghdad library destroyed much of the knowledge of the medieval and ancient East, which Arab Muslims had helped preserve.
Since Islam makes no distinction between secular and religious authority, the Mongol conquests created new debates over the legitimacy of rulers in Muslim thought, as it marked the first time non-Muslims ruled the Muslim world. Perhaps the two most important figures in this debate were al-Ghazali and Ibn-Taymiyyah. The former was a pragmatist who believed that Mongol tyranny was preferable to anarchy and thus advocated political quietism among Muslims unless they were denied the right to practice their faith. On the other hand, the latter had more extreme views and advocated a militant overthrow of the Mongols and restoration of Muslim rulers. These two conflicting ideas both hindered the ability of the Muslim world to develop new ideas of how they could develop newer, more progressive, intellectual and political institutions. The Middle East has still not fully recovered from these developments.
For Eastern Europe, the implications of the Mongol invasions had crushing effects on what could have been considered a primitive form of democracy. Many of the nations of medieval Eastern Europe, most notably Poland and the Republic of Novgorod, which would eventually become part of Russia, had a special type of popular assembly called a veche which played a major role in the legislative and judicial affairs of these countries. Although it is disputed how much these veches were truly democratic or if they were more to provide an illusion of popular choice in public policy, they were nonetheless more progressive than similar assemblies in other parts of Europe.
The Mongol conquest of Eastern Europe destroyed these institutions, and the Mongols replaced the limited form of democracy with their preferred authoritarianism. After the Mongol Empire fell in 1368, the pre-Mongolic political structures of Eastern Europe were unsustainable, and much of Eastern Europe would remain feudal in practice, if not in theory until the mid-20th century.