The enemy decided to keep prisoner the high-ranking officers they had captured.

keep 在这里是动词，意思是"保留、维持某种状态"。 prisoner 是名词，指"囚犯、俘虏"，来自动词 imprison（监禁）的名词形式。 keep prisoner 是一个动词短语，意思是"扣押、囚禁"，指将某人作为俘虏关押起来。