Tumu Crisis (1449) - Eighty years after being kicked out of China, the Mongols returned to take back what they had lost.  In July 1449, 20,000 Mongol soldiers (estimated) under the command of Esen Taishi crossed into Northern China and marched on the city of Datong.  On August 3rd, the Mongols defeated the Ming army garrisoned in the region.  Officials in Beijing (the capital had been moved from Nanjing by then) were surprised by the events.  They quickly assembled a massive army of about a half-million soldiers, led by the emperor himself, and hastily marched out to meet the enemy.  Despite having superior numbers, the Ming army was poorly equipped, disorganized and the march was bogged down by inclement weather.  It reached Datong on August 18th and found the city abandoned.  Reports indicate that the Mongols fled the Ming advance and were returning to Mongol territory.  The emperor declared the expedition a success, re-garrisoned the city and turned back toward Beijing on August 20th.  A week later, the Mongols attacked the rearguard of the Ming army and took it completely by surprise.  The rearguard was annihilated.  When the main army learned of the attack, the emperor dispatched a new force to meet the Mongols.  It too was defeated.  Still confident of his superior numbers, the emperor ignored calls by his generals to camp within a fortified city.  He camped his army near a river to his south, counting on it to be a barrier for his southern flank.  The Mongols diverted the river and managed to completely surround the larger Ming army.  On September 1st, the Mongols won a decisive victory and almost completely wiped out the enemy army.  The emperor was captured in the process.

At this point the Ming Dynasty could have potentially fallen.  The capital was undefended and Esen Taishi planned to attack it.  However, a Ming general in Beijing named Yu Qian managed to rally enough forces to defend an already well-fortified capital, and Esen abandoned the idea.  Instead, he tried to ransom the emperor.  But the emperor's younger brother took advantage of the situation by declaring himself emperor and had no interest in saving his brother.  Four years later he was released without the Mongols having received anything for him.  He was immediately placed under house arrest by his brother.  In 1457, though, he managed to overthrow his brother and return to power.  The Tumu Crisis marked the end of Ming expansion, though it remained a powerful empire for another two centuries.