the river and construct pontoon bridges. His forces crossed the river that night and built camps on the south bank. On the same night, the coalition attacked but were driven back by ambush forces deployed earlier by Cao Cao. Ma Chao and his allies also garrisoned at the south bank and they sent a messenger to meet Cao Cao, requesting to give up the territories west of the river in exchange for peace, but Cao Cao refused. In late October or November 211, Cao Cao's forces attempted to cross the Wei River again, but came under attack by Ma Chao and his cavalry each time they tried to cross. Their camps were not stable and they could not build ramparts because the terrain was too sandy. Lou Gui suggested to Cao Cao: "The weather is cold. We can mix sand with water and use the mixture to construct walls. They will be ready after one night." Cao Cao heeded Lou Gui's suggestion and ordered his troops to work through the night to build the walls, which were ready by the following day. Cao Cao and his forces were hence able to cross the Wei River while the enemy was kept at bay by the walls. Ma Chao led his men to attack the walls but were repelled by Cao Cao's ambushes. However, there have been suspicions that the building of the sand walls did not actually occur, because the incident took place in late autumn or early winter (late October to November) and the weather was probably not cold enough to cause water to freeze. The fifth-century historian Pei Songzhi commented that the Wei Shu stated that Cao Cao's army reached Tong Pass in late August or September 211, and crossed the Wei River to the north bank in late September or October. The weather could not possibly be that cold in late autumn (late September to October) to cause water to freeze. Cao Cao sowing discord between the coalition members Ma Chao and the coalition repeatedly challenged Cao Cao to come out and engage them in battle but Cao ignored them. The rebels then offered to cede territories and send a hostage to Cao Cao's side in exchange for peace. As suggested by Jia Xu, Cao Cao pretended to agree to accept their offer. Han Sui had a meeting with Cao Cao later. Han Sui's father and Cao Cao