Final answer:
China was first united under one ruler during the Qin Dynasty, established by Emperor Qin Shi Huang between 221 BCE and 206 BCE.
Step-by-step explanation:
China was first united under one ruler, an emperor, during the Qin Dynasty. The Qin Dynasty was established when Emperor Qin Shi Huang successfully united seven warring states and formed a centralized empire. This momentous event occurred from 221 BCE to 206 BCE, marking the start of imperial rule in China that would last until 1912 CE, although the Qin Dynasty itself was relatively short-lived, enduring only 15 years. Despite its brief duration, the Qin Dynasty set numerous standards and systems, such as standardized weights and measures, coins, and the writing system, that would influence the governance of China for centuries to come. Additionally, during this period, remarkable artifacts like the Terracotta Army were created to accompany the emperor in his mausoleum.