evidence: After Tang Taizong defeated the Eastern Turks, the northern reaches of the Tang Empire extended all the way to the area around Lake Baikal. But when the Turks later regrouped, Tang control retreated to the Yinshan Mountains (in today’s Mongolia). Up to the reign of the Yongzheng Emperor during the Qing Dynasty, China’s borders did not include Xinjiang and it was not until the middle of the 18th century that the Qianlong Emperor was able to pacify the regions to the north and south of Tian Shan and extend Qing territory as far as Lake Balkhash and the Pamir Plateau. However, beginning around 1860, the northern reaches of Heilongjiang, the area east of the Ussuri River, and the northwest part of what is today Xinjiang—over 1,000,000 square kilometers of territory—was seized by Russia. In the 20th century, China for all practical purposes lost 1,500,000 square kilometers of territory that is Outer Mongolia.