The Crisis of the Tumu Fortress was a frontier conflict between the Northern Yuan and Ming dynasties. The Oirat ruler of the Northern Yuan, Esen, captured the Emperor Yingzong of Ming on September 1, 1449.

The entire expedition had been unnecessary, ill-conceived, and poorly commanded. The Northern Yuan victory was won by an advance guard of perhaps as few as 5,000 cavalry. Esen, for his part, was not prepared for the scale of his victory or for the capture of the Ming emperor. At first he attempted to use the captured emperor to raise a ransom and negotiate a favourable treaty including trade benefits. However, his plan was foiled in the Defense of Beijing due to the steadfast leadership of the Ming commander in the capital, General Yu Qian. The Ming leaders rejected Esen's offer, Yu stating that the country was more important than an emperor's life.

The Ming never paid a ransom for the return of the Emperor, and Esen released him four years later. Esen himself faced growing criticism for his failure to exploit his victory over the Ming and he was assassinated six years after the battle in 1455.