No one can tell precisely when the building of the Great Wall was started but it is popularly believed that it originated as a military fortification against intrusion by tribes on the borders during the earlier Zhou Dynasty. Late in the Spring and Autumn Period (770 BC - 476 BC), the ducal states extended the defence work and built "great" structures to prevent the attacks from other states. It was not until the Qin Dynasty that the …showed first 75 words of 545 total…

…showed last 75 words of 545 total…this giant dragon. For centuries, the wall served succeeding dynasties as an efficient military defence. However, it was only when a dynasty had weakened from within that invaders from the north were able to advance and conquer. Both the Mongols (Yuan Dynasty, 1271-1368) and the Manchurians (Qing Dynasty, 1644-1911) were able take power because of weakness of the government and poverty of the people but never due to any possibility of weakness of the Wall.