The correct answer would be the border dividing the Chinese Han Dynasty and Xiongu.
In 200 BC, the Chinese Han Dynasty invaded Xiongu territory in order to take it over and control it. However, they were defeated, and had to withdraw from the battle.
On the other hand, there were also attacks from Xiongu in Chinese territory. After Han's defeat in Baideng, a peace treaty was reached. This conflict is known as The Battle of Baideng.
