 Complete the following phrase:  Chao was one of the first known ministers to suggest to Emperor Wen that Han armies should have a cavalry-centric army to match the nomadic Xiongnu to the north, since Han armies were still primarily infantry, with cavalry and chariots playing a supporting role. He advocated the policy of using barbarians to attack barbarians, that is, incorporating surrendered Xiongnu horsemen into the Han military, a suggestion that was eventually adopted, especially with the establishment of dependent states of different nomads living on Hans frontiers. Like Jia Yi 201168 BC, he was an early proponent of terminating the heqin marriage alliance and tribute treaty with the Xiongnu, although he opposed it for practical reasons rather than Jias staunch ideological position that superior sedentary Chinese culture should dominate over the northern nomads. The heqin agreement was supposed to guarantee peace and stability between the Han and Xiongnu. Chao believed that the heqin agreement had been breached and ignored by the Xiongnu on so many occasionswith continuous raiding and plundering along Hans borders by Xiongnu tribe and clan leadersthat the treaty simply failed to live up to its goals and lost all practical use. It was not until after the Battle of Mayi 133 BC during Emperor Wus reign 14187 BC that the heqin treaty was finally abolished in favor of 