Modu Chanyu
In 177 BCE, the Xiongnu Wise King of the Right raided the non-Chinese tribes living under Han protection in the northwest (modern Gansu). In 176 BCE, Modu Chanyu sent a letter to Emperor Wen informing him that the Wise King, allegedly insulted by Han officials, acted without the Chanyu's permission and so he punished the Wise King by forcing him to conduct a military campaign against the nomadic Yuezhi. Yet this event was merely part of a larger effort to recruit nomadic tribes north of Han China, during which the bulk of the Yuezhi were expelled from the Hexi Corridor (fleeing west into Central Asia) and the sedentary state of Loulan in the Lop Nur salt marsh, the nomadic Wusun of the Tian Shan range, and twenty-six other states east of Samarkand (Sogdia) were subjugated to Xiongnu hegemony. Modu Chanyu's implied threat that he would invade China if the "heqin" agreement was not renewed sparked a debate in Chang'an; although officials such as Chao Cuo and Jia Yi (d. 169 BCE) wanted to reject the heqin policy, Emperor Wen favored renewal of the agreement. Modu Chanyu died before the Han tribute reached him, but his successor Laoshang Chanyu (174–160 BCE) renewed the heqin agreement and negotiated the opening of border markets. Lifting the ban on trade significantly reduced the frequency and size of Xiongnu raids, which had necessitated tens of thousands of Han troops to be stationed at the border. However, Laoshang Chanyu and his successor Junchen Chanyu (r. 160–126 BCE) continued to violate Han's territorial sovereignty by making incursions despite the treaty. While Laoshang Chanyu continued the conquest of his father by driving the Yuezhi into the Ili River valley, the Han quietly built up its strength in cavalry forces to later challenge the Xiongnu.