Who was the Great Wall of China built to keep out?

The Great Wall safeguarded the central plain of Mainland China in history and successfully kept out invasions from various northern nomadic tribes including the Huns in the Qin and Han Dynasties, the Turks in the Sui Dynasty, the Khitan in the Song Dynasty, and the Tatar, Oirat and Jurchen in the Ming Dynasty.

For thousands of years, it acted like a patron saint but meanwhile a barrier for the power extension of Han Chinese who ruled the central plain. In all of ancient Chinese history no Han ruled north of the Great Wall.
While the Wall kept out the invaders, it did not block cultural exchange and national integration. Han people in the central plain and nomads in the north traded frequently and many nomads moved south and gradually became part of the Han people. For instance, Emperor Gaozong of the Tang Dynasty is not a pure Han, for his mother is a Xianbei woman.
 Did the Great Wall always work in resisting the enemies? Most of the time, it functioned as a solid defensive line and successfully kept out the northern invaders. It failed to do so only once in over 350 cold steel battles in its 2,700 years’ history. The exception was Genghis Khan! In 1213 AD, the Mongols, led by Genghis Kahn, broke through the Juyongguan Pass near Beijing. The Mongol later overthrew the Jin Dynasty (1115 - 1234 AD) and set up the vast and powerful Mongol Empire (1206 - 1368 AD).

The Jurchen or Manchu, the founder of the Qing Dynasty (1644 – 1911 AD), also entered the central plain through the Great Wall, but it was not a “breakthrough”. Actually the Ming General, Wu Sangui, switched sides and opened the gate of the Shanhaiguan Pass to let the Manchus in.
 

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Different groups of northern nomadic tribes grew, developed, immigrated or melded with others in history, so the Great Wall kept out different northern nomadic tribes at different times as follows:
 The Great Wall of China was first built by various states during the Zhou Dynasty to block invasions from neighboring states. For instance, the Qi State built the wall to block the Chu State, and the Chu built it to block the Qi and Qin States.
 Beginning with the Qin Dynasty, it was mainly used to keep out nomadic invaders. After Emperor Qin Shi Huang unified the central plain and set up the Qin Dynasty (221 - 207 BC), the Huns, or Xiongnu, grew strong and often ran across the northern border to rob the people, so the emperor directed that the existing walls of different states be linked to block them.

The Huns were still a threat in the Han Dynasty (202 BC - 220 AD), so the rulers used the previous Qin Great Wall and built some new sections to keep them out.
 Many historians think the Jin Dynasty is one of the dynasties in China history that did not help build the Great Wall. Actually, early in this period, the Xianbei frequently attacked the northeast border and the previous Qin Dynasty Great Wall was reinforced to keep them out.
 The Northern and Southern Dynasties was a tumultuous period when regimes changed frequently and a few countries even coexisted at the same time in the central plain. Afraid of attacks from other countries, defensive sections were added in different areas. In addition, the powerful nomadic tribes in the north, mainly including Rouran, Turk (Tujue), and Khitan (Qidan), were also those the Great Wall aiming to keep out.
 In the Sui Dynasty (581 - 618 AD) and the early Tang Dynasty (618 - 907 AD), the Turks were the biggest threat. The Emperor Yang of Sui commanded that the Great wall be built extensively along the northern border to keep them out. In the early Tang Dynasty, the previous walls were still in use. Later, as the Tang became stronger and stronger, the northern nomadic tribes were not really threats at all.
 In the Song Dynasty, the northern nomadic groups became so powerful that they founded their own empires called Liao Dynasty (907 - 1125 AD) of Khitan, Western Xia Dynasty (1038 - 1227 AD) of Tangut, and Jin Dynasty (1115 - 1234 AD) of Jurchen, respectively. Some sections of Song Great Wall were built more in the south because the Jurchen narrowed the Song territory to the south of Yangtze River.
 The Jin Dynasty built the Great Wall twice: once from 1194 to 1201 to keep out the Tangut people and other small nomadic tribes; and again from 1210 to 1211 to keep out the Mongols. However, the Jin Great Wall did not keep out the Mongols, who broke through it and set up Yuan Dynasty (1271 - 1368 AD).
 

After the Mongols were driven back to the northern grassland, they divided into two groups, Tatar and Oirat, and kept attacking the northern border. Meanwhile, the Jurchen, later known as Manchu, in the northeast grew stronger and became a big threat, so the Ming Great Wall was built to protect the territory.


Famous Great Wall Battles:

 General Meng Tian Attacked the Huns in the North

 Wei Qing and Huo Qubing Beat Back the Huns

 Genghis Khan Breached China Great Wall

 Tumu Crisis

 The War of Shanhaiguan Great Wall

 Battle of Rehe - Last Battle on the Great Wall