WHEN THE GLORY of T'ang ended in military collapse there followed half a century of dynastic usurpations: the Five Dynasties, in reality no more than military dictatorships that controlled only part of North China. When China was again united under one dynasty, the Sung Dynasty (A.D. 960-1279), it was a much smaller empire and a more inward-looking country that emerged from the tribulations and agonies of the decades of the last years of T'ang and the interregnum. By the time of unification under the Sung, not only was a great part of the empire lost to nomad invaders from the steppes of Central Asia and the forests of Manchuria, but even a part of China Proper south of the wall remained in the hands of these intruders. The remaining smaller empire was continuously threatened by invaders, and eventually the Mongols under Genghis Khan and his successors conquered it all.