We must know that Ma Chao is Ma Teng's most favored son, capable of fighting, commanding troops, with outstanding strength, basically invincible, and even became the King of Qiang. Though Ma Teng does not say it, he naturally feels that the Ma Clan's rise depends on Ma Chao, but then his son ended up in a vegetative state...
An angry Ma Teng gathered all his troops and was ready to fight to the death with Lyu Bu. Before engaging Lyu Bu, he took Zhong Yao's advice to inform Cao Ren. Regardless of whether Cao Ren sent troops or not, Ma Teng had already planned to fight external and internal battles!
At this point, Cao Ren naturally wouldn't refuse. Ma Teng's proposal was exactly what he wanted; Cao Cao originally had the idea of using the Xianbei tribe to weaken Lyu Bu and then absorbing Lyu Bu's forces strongly.
