After his contract ended, he decided to reenlist in the army.

reenlist 是由前缀 re-（表示"再次"）和动词 enlist（入伍、参军）组成的复合动词，字面意思是"再次入伍"。enlist 本身由 en-（使进入某种状态）和 list（名单、登记）组成，原指"登记入伍"。