...for the Northern Sung the only reliable source of cavalry mounts came to be the high grasslands of northeastern Tibet (the modern Chinese province of Qinghai)... (pg. 26)With good grasslands, a surplus of horses, and armies of 60,000-100,000 troops, the military strength of the two major Tibetan federations was impressive. But the main source of their wealth and power was the profits they accrued as participants and middlemen in the Inner Asian trade. Because of their position at the eastern terminus of the Gansu corridor, virtually all of the trade routes linking Turkestan and Song China passed through Tibetan domains and depended on Tibetans for protection against raids by Xi Xia and the smaller steppe tribes... (pg. 27)
The Sino-Tibetan alliance of trade and mutual defense was the Song answer to the broad structural challenge of procuring horses... policy of expanding the frontier, forcibly annexing the Tibetan tribal lands of the northwest, and subsequently establishing a state-run enterprise that nestled markets in the new territories and paid for Tibetan horses with Sichuanese tea... (pg. 30)