The Battle of Tong Pass (潼关之战) was fought in 211 when Ma Chao and Han Sui led a coalition of northwestern warlords against Cao Cao. Cao Cao had summoned Ma Teng to the capital, which contributed to Ma Chao’s decision to rebel. The coalition assembled at Tong Pass (潼關), the strategic gate into Guanzhong. Cao Cao held the pass and then sent a force across the Yellow River to threaten the coalition’s rear, drawing them west. He also used a stratagem: he wrote a letter to Han Sui that was deliberately smudged or altered so that Ma Chao would think Han Sui was secretly dealing with Cao Cao. The coalition split; Han Sui made peace with Cao Cao, and Ma Chao was defeated and fled. The battle secured the Guanzhong region for Cao Cao and broke the northwestern alliance.
Context
Ma Teng had been summoned to Ye and was effectively a hostage; Ma Chao and Han Sui raised troops and were joined by other local leaders. Tong Pass was the choke point between the northwest and the Central Plain. Cao Cao’s crossing of the river and the letter ruse are among the most cited examples of his tactical and psychological warfare.