211 AD. Cao Cao announced that he was invading 张鲁 at 汉中. He had to pass through the 关中 region to get there. 马超 figured that Cao Cao’s real intention was to invade 关中. He hastily formed an alliance with his father’s friend 韩遂 to attack Cao Cao’s forces.
The 关中 region has a natural barrier in the form of the 华山 range. The most direct route was through 潼关. Cao Cao ordered his generals to form up at 潼关, ready to cross the Yellow River. Ma Chao and Han Sui responded by building a strong defence on the southern bank of the river. Cao Cao had a problem. He needed a 3:1 majority to overwhelm the defenders and he had only about the same number of soldiers as the other side.
In this region, the Yellow River forms an L shape, flowing from north to south, then from west to east. 潼关 is situated on the East-West portion. Cao Cao then did something unexpected. He led a small force himself and crossed the Yellow River on the North-South section, ending up at 大荔。From there to 关中, there was still the 渭水 river to cross. Han Sui laughed at the seemingly foolish manoeuvre and wanted to block Cao Cao’s supply route. Ma Chao decided to finish him off while he was crossing 渭水. Cao Cao took a gamble and survived the attack.
With the two leaders distracted, Cao Cao’s main force attacked 潼关 and Ma Chao’s army was in disarray. Ma Chao and Han Sui offered an armistice, ceding land to Cao Cao letting him hold his son as hostage in his camp. Cao Cao knew it was a trick. He invited Han Sui alone to his camp. At the meeting, Cao Cao did not make any negotiations at all. Instead, it was all casual talk with the two men recounting stories from their younger days.
Han Sui returned to camp drunk. When Ma Chao asked him what was discussed, Han Sui told the truth but aroused Ma Chao’s suspicion. Later, Cao Cao wrote a letter to Han Sui and brushed over a few lines, rendering some parts illegible. Ma Chao asked Han Sui why he had censored the letter from Cao Cao. Han Sui said that it was Cao Cao who did it. Ma Chao was not convinced. That would be the last straw. The two men fought and Cao Cao came up from behind, decimating the Ma-Han alliance.