Answer:
Sent princesses to Marry Xiongun leaders.
Step-by-step explanation:
At first, China tried to block the advance of Xiongnu Empire by miltary force, but they failed and they had to use one of their last resorse tactics: Arrange marriages with the enemy. They sent princesses to Marry Xiongnu important leaders along with gifts to ecourage them to stop their attacks; even the daugther of the emperor was offered for marriage. It was useful to set a peace treaty between the Chinese and the Xiongnu which kept the Xiongnu as allies for decades, however it was very costly for China as they had to sent gifts constantly and accept trading conditions more benefical for the Xiongnu. At the endgame, the peace was broken when the Xiongnu attacked again now stronger in power and trade.