In 117 BC, after three large-scale campaigns during Emperor Wu of Han's reign, the Xiongnu's power was greatly weakened, forcing them to flee further north into the desert. To protect their people, the Xiongnu decided to seek a truce with the Han Dynasty. Emperor Wu had long desired to subjugate the Xiongnu, so their initiative to negotiate peace was ideal. However, as soon as talks began, Emperor Wu's anger flared—the Xiongnu audaciously demanded that the Han Dynasty annually supply them with grain, gold, and treasures, threatening that if refused, the Han envoys still held captive among them would never return to the Central Plains. Emperor Wu naturally could not agree, and thus the war between Han and the Xiongnu dragged on for another decade.
In 100 BC, a new Chanyu rose to power among the Xiongnu tribes, and Emperor Wu of Han decided to send an envoy to the Xiongnu, both to congratulate the new Chanyu on his ascension and to negotiate an end to the war between the two states. The emperor selected Su Wu, a Palace Attendant General, as his envoy, and so Su Wu set off with over a hundred men and a great quantity of treasures, advancing in grand style toward the barren desert.
After completing his diplomatic mission, just as Su Wu was preparing to return to Han, internal turmoil broke out among the Xiongnu, and Su Wu and his companions were temporarily detained. The Chanyu, knowing Su Wu was a loyal official, wanted him to stay with the Xiongnu forever, so he tried to persuade Su Wu to abandon the Han dynasty. When Su Wu refused, the Xiongnu promised him high official positions and titles, along with generous rewards of wealth, all of which Su Wu rejected. Enraged, the Chanyu locked Su Wu in an open pit in the ground. In the bitter northern winter, large snowflakes fell from the sky, and the piercing wind seemed to cut through flesh and penetrate the marrow. To force Su Wu to submit, the Chanyu ordered that he be given no food or water. Facing such cruel conditions, Su Wu held firm to his convictions and refused to yield to the Xiongnu. When thirsty, he grabbed handfuls of snow to eat; when hungry, he tore wool from his sheepskin coat and chewed on it. With nothing to shield him from the wind in the pit, he curled up in his wool coat at night to sleep. Even so, Su Wu never bowed to the Xiongnu. After several days, seeing Su Wu barely alive, the Chanyu grew even more admiring and determined to keep him. He first released Su Wu from the pit, then gave him a flock of sheep and exiled him to the North Sea (present-day Lake Baikal in Siberia) to tend them, hoping to break his spirit. The Chanyu told Su Wu that he could return to his homeland only when the rams gave birth to lambs.
Su Wu led his flock of sheep to the desolate Lake Baikal, where there was not a single farmhouse in sight, and he discovered that all the sheep were male, with no females, making it impossible for lambs to be born, let alone for the rams to produce offspring. The Chanyu intended to drive Su Wu into utter despair and extinguish any thought of returning to Han. With no one to help and no way to escape this place alone, Su Wu had to survive by drinking melted snow and catching wild rabbits, herding the sheep by the lake every day while clutching tightly the ceremonial staff that symbolized his status as an envoy of the Han Dynasty, and sleeping among the flock at night. As years passed, the tassels on the staff wore out and fell off completely, and Su Wu's hair and beard turned white, but he never forgot for a single day that he was a Han envoy, firmly believing that one day he would return to his homeland.
Su Wu stayed by Lake Baikal for nineteen years. During those nineteen years, many things happened: the Xiongnu chieftain who had detained him died, and his son became the new chieftain; the political situation in the Han Dynasty also changed—Emperor Wu of Han died, and Emperor Zhao of Han ascended the throne. After the new chieftain took power, he immediately sent envoys to seek peace with the Han Dynasty, and Emperor Zhao of Han demanded that all envoys detained in Xiongnu over the years be released first.
To demonstrate sincerity, the Xiongnu indeed released all the detained Han Dynasty envoys, and Emperor Zhao of Han sent an official to the Xiongnu to escort the envoys back to court. The official did not see Su Wu among the crowd and demanded his return from the new Chanyu, who claimed Su Wu had died. The official was suspicious but had no evidence to prove the Chanyu was lying, so he had to drop the matter. On the night before the official and envoys were to leave the Xiongnu, an envoy who had been detained secretly bribed a servant close to the Chanyu and learned that Su Wu had been exiled to herd sheep on Lake Baikal. The envoy quickly informed the official. The official immediately confronted the Chanyu, saying that Emperor Zhao of Han had recently been hunting at the Shanglin Garden when a wild goose suddenly flew down and landed before the emperor, with a piece of cloth tied to its foot bearing the message that Su Wu was herding sheep on Lake Baikal. The Chanyu was astonished, never imagining that Su Wu could receive help in such harsh conditions to send a distress signal, and believing that Su Wu's loyalty had moved heaven and earth, he sent people to bring Su Wu back. When everyone saw Su Wu with his white hair, they could not help but weep.
In 81 BCE, after nineteen years as an envoy to the Xiongnu, Su Wu finally returned to Chang'an, and his unyielding spirit deeply moved Emperor Zhao of Han, who rewarded him with many gifts. The following year, Shangguan Jie, Shangguan An, and Sang Hongyang launched a rebellion in an attempt to overthrow the emperor, but Emperor Zhao suppressed it; Su Wu's son was implicated in the revolt, leading to Su Wu's dismissal from his official post. After Emperor Zhao's death, Su Wu and several court officials supported Emperor Xuan's ascension to the throne, and Su Wu was subsequently enfeoffed as the Marquis of Guannei by Emperor Xuan.
In 60 BC, Su Wu passed away. His loyalty to the nation will forever be remembered and learned by future generations.