Records of the Three Kingdoms
Ma Su, courtesy name Youchang, was from Yicheng in Xiangyang. His elder brother Ma Liang died in the Yiling campaign. Zhuge Liang singled him out for talk of military plans. On his deathbed Liu Bei said Ma Su’s words exceeded the facts and he should not be given a great command; Zhuge Liang was to watch him. That warning is in Ma Su’s biography.
Before the southern campaign of 225, Pei quotes the Xiangyang ji: Ma Su told Zhuge Liang that the way of war is to attack the heart first. Chen Shou’s account of the campaign itself is short. In 228 Zhuge Liang came out at Qishan. He did not give the van to the veterans Wei Yan or Wu Yi. He sent Ma Su to command the forward armies against Zhang He at Jieting. Ma Su disobeyed the deployment, mishandled the position, and was routed. With Jieting lost, Zhuge Liang had no base to advance from. He retreated and executed Ma Su. Ma Su was thirty-nine. Zhuge Liang demoted himself three degrees. Jiang Wan later asked whether it was a pity to kill a man of plans while the realm was unsettled. Zhuge Liang said that without military law no one would obey again.
Romance of the Three Kingdoms
The novel’s Jieting has Ma Su reject Wang Ping’s advice, camp on a hill, and lose the water to Zhang He. The Shu army collapses without a real fight.
Zhuge Liang executing Ma Su in tears is the climax: the law on one side, weeping on the other. Liu Bei’s words at Baidi are planted earlier as “Ma Su must not be trusted with much.” The tears turn a piece of military law into a scene of character.
Reference: Wikipedia · Ma Su