Battle of Guandu
In 200, Cao Cao was locked in a stalemate against Yuan Shao at the Battle of Guandu for months, eventually exhausting his food supply; while contemplating retreat he sent a letter to Xun Yu (who was then defending Xu) for advice. Xun Yu dissuaded Cao Cao with a letter, highlighting several advantages that his army held over Yuan Shao's forces and urging him to stand fast; the eventual result was a decisive victory for Cao Cao, which was crucial to his domination of northern China.