In the mid 7th century CE a young girl was brought in the harem of the current Chinese emperor. A year later he died and all the members of the harem are automatically ‘retired’ to a type of nunnery. The new emperor met the young girl and fell in love with her and eventually brought her to his court and eventually made her the favorite wife. When he died one of his nephews took the throne but then in 690 CE Wu Zetian became the one and only female ruler of China. She ruled for 15 yeas before abdicating (peacefully) to one of the nephews she had taken over from. Later chroniclers of Chinese history have described her reign in bad terms but it appears to current historians that they just did not approve of a woman ruler. Her rule seems to have been peaceful and of no great significance good or bad.
(The Great Courses – From Yao to Mao: 5000 Years of Chinese History)