In 1231, the Mongols began their invasions of Korea during seven major campaigns and 39 years of struggle, but were unable to conquer Korea. Exhausted after decades of fighting, Goryeo sent its crown prince to the Yuan capital to swear allegiance to the Mongols; Kublai Khan accepted, and married one of his daughters to the Korean crown prince, and for the following 80 years Goryeo existed under the overlordship of the Mongol-ruled Yuan dynasty in China. The two nations became intertwined for 80 years as all subsequent Korean kings married Mongol princesses, and the last empress of the Yuan dynasty was a Korean princess.