在战争中，两国士兵成为了彼此的死敌。

During the war, soldiers from both countries became each other's worst ___enemies.

我们的军队成功击退了敌人的进攻。

Our army successfully repelled the ___enemy's attack.

压力是心理健康的最大敌人。

Stress is the greatest ___enemy of mental health.