The significant event that marked the beginning of the Great Wall of China's construction was the unification of China under Emperor Qin Shi Huang around 221 BCE. To defend against nomadic invasions and consolidate his control over the vast territories, Qin initiated the construction of wall segments that had been built by earlier states. This early version, primarily made of rammed earth, marked the start of a long history of wall-building that would evolve into the extensive structure seen today, reflecting both defense and imperial power.