sentences:  Sometimes neighboring states were actual protectorates or vassal states over which China exerted large amounts of influence, while in other cases foreign states merely acknowledged China's nominal suzerainty in order to gain access to Chinese trade, which took place through the tributary system.The king of the ancient state of Qin first unified the Chinese empire in 221 BC by conquering all of the other states in what was then considered China and proclaimed himself the "First Emperor" and became known as Qin Shi Huang.The ancient Han dynasty (202 BC – 220 AD) established control over northern Vietnam, northern Korea, and the Tarim Basin of Central Asia.