evidence: Just outside of Beijing, the President will tour a section of the Great Wall of China, which stretches over 3,000 miles from China's border with North Korea on the Yalu River, to the foot of the Qilian and Tainshan mountains in China's westernmost region. The Great Wall began as a series of unconnected protective walls constructed by rival kingdoms in ancient China. When Qin Shihuang unified China in 221 B.C., he ordered the walls of his defeated rivals be connected. In the centuries that followed, the Great Wall has expanded, ultimately reaching the length we know today.