误Their duty is to defend the enemy.
正Their duty is to defend themselves against the enemy.
析defend含义是“保卫自己,免受敌人攻击”,其宾语是保护的对象; against后接防御的对象。
她保护她的孩子不被疯狗咬伤。误She defended her children off the mad dog.
正She defended her children from the mad dog.
析表示“保护…使免遭危害”时, defend常与from连用。