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http://en.wikipedia.org/wiki/Empress_Chonchu (Yes I know it's about the drama about the empress not the empress herself but English wiki doesn't have an article about the latter)
Empress Chonchu of Goryeo planned as dowager empress to attack the Khitan and retake Manchu lands of old Gorgueryeo in alliance with Song Dynasty China. However due to an earthquake this attack was cancelled. So what if the earthquake had not happened and Goryeo had struck the Khitan?