King Cheng of Chu struck at the State of Song, the ally of Jin most accessible from the south, in the winter of 633 BCE. In retaliation, an expeditionary force under Duke Wen marched south in the spring of the following year and occupied the States of Wei and Cao, both satellites of Chu. The two sides sought out alliances in the following months. The States of Shen, Xi, Chen and Cai, all immediately contiguous to Chu, sided with King Cheng, as well as the more distant State of Lu.