Complex sentence: The Han Chinese Ming general Wu Sangui and his army then defected to the Qing and let them through Shanhai pass, and then seized control of Beijing, overthrowing Li's short-lived Shun dynasty.
Split into simpler sentences: The Han Chinese Ming general Wu Sangui and his army then defected to the Qing and allowed them through Shanhai pass. They then seized control of Beijing, overthrowing Li's short-lived Shun dynasty.