 Complete the following phrase:  After a period of relative disunity, China was reunified by the Sui dynasty in 581 and under the succeeding Tang dynasty 618907 China entered a Golden Age. The Tang Empire competed with the Tibetan Empire 618842 for control of areas in Inner and Central Asia. The Tang dynasty eventually splintered, however, and after half a century of turmoil the Song dynasty reunified China, when it was, according to William McNeill, the richest, most skilled, and most populous country on earth. Pressure from nomadic empires to the north became increasingly urgent. By 1142, North China had been lost to the Jurchens in the 