Various peoples also expanded and contracted later in history, including the Magyars in the Early Middle Ages, the Mongols and Seljuks in the High Middle Ages, the Kalmuks and the Kyrgyz and later the Kazakhs up to modern times. The earliest example of an invasion by a horse people may have been by the Proto-Indo-Europeans themselves, following the domestication of the horse in the 4th millennium BCE (see Kurgan hypothesis). The Cimmerians were the earliest invading equestrian steppe nomads that are known in Eastern European sources. Their military strength was always based on cavalry, and they were among the first to have developed true cavalry. Historically, areas to the north of China including Manchuria, Mongolia and Xinjiang were inhabited by nomadic tribes. Early periods in Chinese history involved conflict with the nomadic peoples to the west of the Wei valley. Texts from the Zhou dynasty (c. 1050–256 BCE) compare the Rong, Di and Qin dynasty to wolves, describing them as cruel and greedy. Iron and bronze were supplied from China. An early theory proposed by Owen Lattimore suggesting that the nomadic tribes could have been self-sufficient was criticized by later scholars, who questioned whether their raids may have been motivated by necessity rather than greed. Subsequent studies noted that nomadic demand for grain, textiles and ironware exceeded China's demand for Steppe goods. Anatoly Khazanov identified this imbalance in production as the cause of instability in the Steppe nomadic cultures. Later scholars argued that peace along China's northern border largely depended on whether the nomads could obtain the essential grains and textiles they needed through peaceful means such as trade or intermarriage. Several tribes organized to form the Xiongnu, a tribal confederation that gave the nomadic tribes the upper hand in their dealings with the settled agricultural Chinese people. During the Tang dynasty, Turks would cross the Yellow River when it was frozen to raid China. Contemporary Tang sources noted the superiority of Turkic horses. Emperor Taizong wrote that the horses were "exceptionally superior to ordinary [horses]". The Xiajiasi (Kyrgyz) were a tributary tribe who controlled an area abundant in resources like gold, tin and iron. The Turks used the iron tribute paid by the Kyrgyz to make weapons, armor and saddle parts. Turks were nomadic hunters and would sometimes conceal military activities under the pretense of hunting. Their raids into China were organized by a khagan and success in these