Soldiers kill enemies to protect their country. But killing is against the fifth commandment. How do we explain this?

This question concerns the Fifth Commandment and legitimate self-defense. The Catechism speaks about it in sections 2263-2265 (but 2266-2269 are also helpful).

If you are in a situation where someone is threatening your life, you are allowed to defend your life. In other words, you are allowed to fight back. Your life has value and dignity, and you have the right to defend it. So if someone is putting you in mortal danger, then in self-defense you might end their life.

However, there are a few conditions here.

• You have to fight back with proportional strength. So if somebody hits you, that doesn’t mean you can escalate the level of violence. If somebody is threatening to kill you, then you are able to respond proportionally.
• You also can’t want that person to die. Your intention should first and foremost be to defend your own life–the other person dying is basically an unwanted consequence of you defending yourself.
• It needs to be the last possible option. If you can reasonably defend your life without ending someone else’s, you should do it.

When it comes to soldiers in war, we can think of these principles as self-defense on a national scale. The Catechism says the following: “Legitimate defense can be not only a right but a grave duty for someone responsible for another’s life. Preserving the common good requires rendering the unjust aggressor unable to inflict harm. To this end, those holding legitimate authority have the right to repel by armed force aggressors against the civil community entrusted to their charge” (CCC 2265).

This fits into what is known as just war theory. The idea is that a community or nation might be justified in going to war, as long as it is for the legitimate self-defense of their citizens (whom soldiers have a solemn duty to protect).

However, similar conditions apply to this kind of warfare on a large scale as on a small scale. The nation can’t be the aggressor, and must fight back with proportional strength. And taking lives should be the last resort: if there is a way to end the conflict without death (for instance by merely wounding or capturing the enemy), that should be the preferred method. Remember, the goal is to make the aggressor “unable to inflict harm,” in order to protect lives.

So in a situation like this, a soldier in combat is not committing a sin by killing an opponent–but it must be a legitimate, justified case of defense (either of self-defense or defense of the nation).