respective lands they had captured from each other during the war. Long before the Tang and Song dynasties, the Chinese had sent envoys into Central Asia, India, and Persia, starting with Zhang Qian in the 2nd century BC. Another notable event in Chinese diplomacy was the Chinese embassy mission of Zhou Daguan to the Khmer Empire of Cambodia in the 13th century. Chinese diplomacy was a necessity in the distinctive period of Chinese exploration. Since the Tang dynasty (618–907 AD), the Chinese also became heavily invested in sending diplomatic envoys abroad on maritime missions into the Indian Ocean, to India, Persia, Arabia, East Africa, and Egypt. Chinese maritime activity was increased dramatically during the commercialized period of the Song dynasty, with new nautical technologies, many more private ship owners, and an increasing amount of economic investors in overseas ventures. During the Mongol Empire (1206–1294) the Mongols created something similar to today's diplomatic passport called paiza. The paiza were in three different types (golden, silver, and copper) depending on the envoy's level of importance. With the paiza, there came authority that the envoy can ask for food, transport, place to stay from any city, village, or clan within the empire with no difficulties. In the 17th century, the Qing dynasty concluded a series of treaties with Czarist Russia, beginning with the Treaty of Nerchinsk in 1689. This was followed up by the Aigun Treaty and the Convention of Peking in the mid-19th century. As European power spread around the world in the 18th and 19th centuries so too did its diplomatic model, and Asian countries adopted syncretic or European diplomatic systems. For example, as part of diplomatic negotiations with the West over control of land and trade in China in the 19th century after the First Opium War, the Chinese diplomat Qiying gifted intimate portraits of himself to representatives from Italy, England, the United States, and France. Ancient India Ancient India, with its kingdoms and dynasties, had a long tradition of diplomacy. The oldest treatise on statecraft and diplomacy, Arthashastra, is attributed to Kautilya (also known as Chanakya), who was the principal adviser to Chandragupta Maurya, the founder of the Maurya dynasty who ruled in the 3rd century BC. It incorporates a theory of diplomacy, of how in a situation of mutually contesting kingdoms, the wise king builds alliances and tries to checkmate his adversaries. The envoys sent at the time