While the phrase “holy war” may be somewhat misleading, the Biblical idea of war is rooted in the notion that God led his people into battle and that certain Old Testament battles were executed as religious acts and judgments. Although it has been suggested that herem was an element of every Biblical war, this is most unlikely, since it was not decreed in every battle by God in the Bible. In the Bible this herem offers a powerful metaphor for God’s might act in salvation history that will one day be finally culminated in the total destruction of all the sinful and rebellious people who defy Him.
The totally destruction of everything by Israel strike most of us as cruel and unusual punishment on certain people. But there are 3 helpful truths to keep in mind. 1) The People of God were His arm in His execution of divine judgment in those times that this was decreed by Him. God was using His people to act for Him and to do His will in those times. Just like God uses the church today to do His will in reaching the lost with the grace of Jesus. 2) This decree and action was intended to remove the pagan idolatry of rebellious and sinful people. If total destruction would not have taken place, than sin and idolatry would creep back into the hearts and minds of God’s chosen people. This is the same picture as the New Testament truth in putting sin to death and not loving the sinful world around us. 3) This declaration of God was to remind His people that the war was not intended for their benefit but for God to be shown Holy and glorified. The same is true for us today, we do not exist for our own pleasure and good will but for the glory and honor of God alone.
I hope this brief explanation can help us understand why God was issuing the total destruction of everything as Joshua was conquering the Promised Land and finding his life plus.