I've been reading through portions of Joshua and Judges. You can't read those books without coming across some horrific war scenes. God's command to them was to utterly stamp out the pagan nations around them. They were to slaughter wholesale every man, woman, child, and sometimes, even animals. How do we reconcile this with the peace- loving Jesus of the New Testament?
I think it's important for us to remember that God's revelation is progressive. He gave covenants before the Law, and the Law before the Gospel. With our Lord is the fullness of the revelation of God. With Him, everything that is prophesied is fulfilled.
In Old Testament times, God chose to do justice through the military conquest of His people, Israel. Crimes were sometimes punishable by death. But with the coming of our Lord, these things are no longer necessary. Now God calls us to love. Part of that command is to walk in the way of nonviolence. We are told to not resist evil and to turn the other cheek. It is our consistent demonstration of love that demonstrates true justice in the world. Even though we will be persecuted, and even killed, for this testimony, it remains the way of the Lord, which makes it the path of the disciple as well.
So what are we to do with these violent passages from the Old Testament? I believe they demonstrate how ruthless we are to be with the enemies of our spiritual lives; that is, with sin. Too often, we play with sin. We treat it like a pet. But it's really an enemy. We need to utterly slaughter it in our lives. Sometimes it will seem as innocent as children, or as benign as animals, but it all has to go. When we see this command being faithfully carried out by Israel, they prospered. When we are faithful to uproot and demolish sin in our lives, we will prosper as well.
One more thing I should note. Israel conquered its enemies through the power of God. They did not have more people, better military training, or superior weapons. In fact, they were terribly outclassed in all of those things. But they relied on God and were obedient in faith. For that, they saw God's victory. We do the same as we follow their example. We are powerless against sin in our own strength. But through the power of God we are mighty. In His strength we are called to nothing less, and nothing else, than complete victory. Let us go forth, then, and conquer!