<P> The history of the Great Wall of China began when fortifications built by various states during the Spring and Autumn (771--476 BC) and Warring States periods (475--221 BC) were connected by the first emperor of China, Qin Shi Huang, to protect his newly founded Qin dynasty (221--206 BC) against incursions by nomads from Inner Asia . The walls were built of rammed earth, constructed using forced labour, and by 212 BC ran from Gansu to the coast of southern Manchuria . </P> <P> Later dynasties adopted different policies towards northern frontier defense . The Han (202 BC--220 AD), the Northern Qi (550--574), the Sui (589--618), and particularly the Ming (1369--1644) were among those that rebuilt, re-manned, and expanded the Walls, although they rarely followed Qin's routes . The Han extended the fortifications furthest to the west, the Qi built about 1,600 kilometres (990 mi) of new walls, while the Sui mobilised over a million men in their wall - building efforts . Conversely, the Tang (618--907), the Song (960--1279), the Yuan (1271--1368), and the Qing (1644--1911) mostly did not build frontier walls, instead opting for other solutions to the Inner Asian threat like military campaigning and diplomacy . </P>

Why is the great wall of china built
find me the text answering this question