Yuan Chonghuan
In particular, the Chongzhen Emperor's execution of Yuan Chonghuan on extremely flimsy grounds was regarded as the decisively fatal blow. At the time of his death, Yuan was supreme commander of all Ming forces in the northeast, and had just rushed from the borders to defend the capital against a surprise Manchu invasion. For much of the preceding decade, Yuan had served as the Ming Empire's bulwark in the north, where he was responsible for securing Ming borders at a time when the Empire was suffering humiliating defeat after defeat. His unjust death destroyed Ming military morale and removed one of the greatest obstacles to the eventual Manchu conquest of China.