During the Tang Dynasty in China, surrounding Asian countries acknowledged China's supremacy by sending tributes. Chang'an, now known as Xi'an, served as the capital of both the Han and Tang dynasties. Empress Wu became the first female ruler of China. Foot binding became common during the Song Dynasty to keep women's feet small. Genghis Khan unified the Mongol tribes and established the largest contiguous empire in world history. Ivan III expanded Russian territory after the decline of Mongol rule and declared himself Czar.