It boils down to the sanctity of life. God demands an account for the lives that are taken.(1)
If you were an Israelite in the Old Covenant, it would make sense to annihilate those who are being judged by God as his enemies.(2) In a sense, God's commandment to destroy those idolatrous people was a preview to the final judgement, when God will utterly destroy the wicked. Was it just for God to destroy the wicked back in the days of Joshua? Of course, in the same way that it will be just for God to destroy his enemies on the day of judgement.
Now, if you are a Christian under the New Covenant, things are different because of the dispensation of God's grace. In fact, God has now opened the door of repentance and faith unto salvation through Jesus Christ, for all peoples and nations of the earth.(3) Jesus came into the world not to condemn the world, but to save the world.(4) He has given us a commission to go and preach this Good News to the entire world until He comes back.(5) When He comes back, then it will be judgment time. Until then, we are not called to take life, but to lay down our own lives for the sake of others, including our enemies.(6)
Instead of being concerned with the interests of the world, we should be concerned with the interests of God. What is more important, making sure that Americans keep their freedom to dress scandalously on the beach and drink themselves to oblivion, or giving up your freedom for the sake of Christ and saving lives through the sacrificial preaching of the Gospel?
We must remember that we do not fight as the world does. On the contrary, our weapons are spiritual,(7) because our enemy is also spiritual.(8) Now, I recognize that God uses the governing authorities even to restrain the evils of the world,(9) but we as Christians live by different standards.
I understand that many will disagree with me. I am not looking for man's approval. However, I will leave you with this counsel: Don't fight against your conscience. Even if you think that going to war in Afghanistan is justified, please do not fight against your conscience if it bears witness against you. The result will be utterly destructive. Time and time again, people are convicted of their sin; they KNOW they have done wrong and murdered men against God's will and their conscience keeps bothering them. It would be unwise to try to persuade them to supress these feelings with drugs and try to convince them that they "have done their duty". Instead, they should acknowledge their sins, confess them, and repent. Only then, can there be forgiveness, cleansing, and healing.(10)
1. Genesis 9:5
2. Deuteronomy 9:3-4
3. 2 Corinthians 6:2
4. John 3:17
5. Matthew 24:14, Acts 10:42, Matthew 28:19
6. Matthew 5:44-48, Matthew 10:39, Matthew 16:25, Romans 12:20-21, Acts 20:22-26
7. 2 Corinthians 10:3-4
8. Ephesians 6:12
9. Romans 13:1-4
10. James 5:16, 2 Corinthians 7:9-10, 1 John 1:8-10