The Mongol Empire was the largest land empire in history, started by Genghis Kahn who united the Mongols and led them to conquer China, central Asia, and Persia. His sons divided the empire into four "khanates" or "ulus", under the Great Kahn Ogedei son of Genghis .
Invasion of Europe
Batu, Kahn of the Golden Horde (Russia), invaded Europe in 1241. He sacked Kraków and defeated the Polish army, but the death of Ogedei in Asia caused him to retreat since all Mongols traditionally needed to be personally present at the election of a new Kahn.  To this day, Poland continues to celebrate the battle, claiming it as a strategic victory because the Mongols retreated.
Life under the Mongols
The Mongol Empire continued for a significant time in Asia. Historians retrospectively term this period the "Pax Mongolica," saying the Mongols curbed tribal warfare.
Kublai Khan, Mongol ruler of China, asked the Pope in Rome to send missionaries to preach the Gospel to his people. The Pope sent two monks with Marco Polo; however, both the monks turned back before reaching the Mongol lands. Disappointed, Kublai turned to Buddhism. Marco stayed in China for a long time, gaining a high governmental office.
The Russians continued for centuries to pay tribute to the Kahnate of the Golden Horde.
- The Collected What If? Eminent Historians Imagine What Might Have Been. Ed. Robert Cowley.
- The Travels of Marco Polo, Marco Polo