suzerain rule of China. Some were direct vassals. Theoretically, the lands around the imperial capital were regarded as "five zones of submission", - the circular areas differentiated according to the strength of the benevolent influence from the Son of Heaven. There were several periods when Chinese foreign policy took on isolationist tones, because of the view that the rest of the world was poor and backward with little to offer. Nevertheless, China was a center of trade from early on in its history. Many of China's interactions with the outside world came via the Silk Road. This included, during the 2nd century AD, contact with representatives of the Roman Empire, and during the 13th century, the visits of Venetian traveler Marco Polo. Chinese foreign policy was usually aimed at containing the threat of so-called "barbarian" invaders (such as the Xiongnu, Mongols, and Jurchen) from the north. This could be done by military means, such as an active offense (campaigns into the north) or a more passive defense (as exemplified by the Great Wall of China). The Chinese also arranged marriage alliances known as heqin, or "peace marriages." Chinese officers distinguished between "matured/familiar barbarians" (foreigners influenced by Chinese culture) and "raw barbarians". In many periods, Chinese foreign policy was especially assertive. One such case was exemplified by the treasure voyages of Admiral Zheng He during the Ming dynasty. Qin Dynasty Although many kings of the Shang and Zhou dynasties ruled beforehand, in 221 BC, the ruler of the Qin state, Ying Zheng (Qin Shi Huang), was the first to conquer the different vassal states under the Zhou dynasty, as well as other non-sinicized states. He was able to transform these different states into a relatively unified and uniform empire, the Qin Empire. Under his leadership and a society modelled around strict adherence to legalist philosophy, his once backwater western frontier state conquered all of the rivaling Warring States in ancient China. The Chinese domain was also extended into Inner Mongolia and Manchuria to the north, and with naval expeditions sent to the south, the indigenous Baiyue of modern-day Guangdong and northern Vietnam (the latter called Jiaozhi, and then Annam during the Tang dynasty) were also quelled and brought under Chinese rule. Han dynasty The time of the Han dynasty (202 BC–AD 220) was a groundbreaking era in the history of Imperial China's foreign relations, during the long reign of Emperor Wu