Final answer:
China was unified under the rule of its first emperor, Qin Shi Huang, in 221 BCE, marking the start of the Qin Dynasty.
Step-by-step explanation:
China became a region of unified states under the rule of its first emperor in the year 221 BCE. The unification was the result of the campaigns led by Emperor Qin Shi Huang, the founder of the Qin Dynasty. This monumental event marked the end of the Warring States period and the establishment of the first imperial dynasty in China. The Qin Dynasty centralized power, standardized various systems such as weights and measures, currency, and the writing system. They also made advancements in governance that laid the foundations for future dynasties, although the Qin Dynasty itself only lasted from 221 BCE to 206 BCE.