During the war, several European countries were occupied by enemy forces.

occupied 是动词 occupy 的过去分词形式，在句中表示被动。occupy 由前缀 oc-（表示"朝向"）和词根 -cupy（来自拉丁语 capere，意为"抓住、占据"）组成，意思是"占领、占据"。occupied 表示"被占领的"状态。