It's difficult to preach pretty much any Old Testament book without coming to the rather vexed question of divine war [Incidentally, Tremper Longman III argues that "divine war" or "Yahweh war" is a better description than "holy way". I agree]. What are we to make of those total destruction passages and, indeed, much of the violence we find in the Old Testament. A recent book by Joseph Smith has ably made the point that this is a consistent theme running through the Scriptures. Christians don't really know what to do with these passages. And neither, often, do preachers.
We know instinctively that the Chalke approach is wrong. Moses and Joshua did not "mishear" Yawheh's voice when he prescribed the kind of warfare that Israel must engage in (e.g. Deut 7 or Deut 20). It's easy to dismiss that kind of insipid Marcionism. But it's not easy to know what to replace it with. Some evangelicals just spiritualise everything: what is true of physical warfare in the OT is true of spiritual warfare in the New. That is right - but only up to a point. There's more, surely, because the final battle that Christ, the divine warrior fights, is hardly "just spiritual" - as we will discover, it is the most physical battle ever fought, winning for us eternal life in the Son's presence, but condemning to eternal physical destruction those who stand opposed to him.
It seems to me the Law passages help us because they explain the background to divine war.
Yahweh is the instigator of war (Deut 7.1), he decides who will be fought. He chooses the battles.
Yahweh is the agent of war (Deut 7.2), he is the one who is doing the fighting. The Israelites must learn to fight in his strength alone.
Yahweh is present in the war (Deut 20.1), his presence is made known by the Ark. The Ark is a physical reminder that this is Yahweh's war.
Yahweh is always victor in the war (Deut 20.4), he delivers enemies to Israel. With Yahweh on side, there is never a battle defeat. Defeats come when Yawheh is absent or - worse - against his people.
Yahweh is executing total judgement in war (Deut 20.18), his war is just. Everybody deserves destruction. Although the warfare to us sounds tyrannical, it is always just.
Yahweh displays grace to Israel in war (Deut 7.7-11), the only reason Israel is preserved. Israel also deserves destruction but is preserved through God's special choosing (though in apostasy is not excluded from God's just fighting).
Those seem lasting principles, even if the nature of the battle changes. But how does it change?
When Christ comes, he is presented as the divine warrior, the one who fulfils militaristic prophecy. But the gospels make clear he has come to wage war against root enemies, rather than physically superficial ones: death, Satan, sin. Those who are in Christ appropriate the victories that Christ has already delivered. But that is not all there is to say. There is still a final battle. There are still prophecies to be fulfilled in Christ and the imagery of Revelation is still physical. Therefore we should think of Christ the divine warrior having unfinished business.
This, then, helps us preach divine war. We need to apply the principles above to the battle we find ourselves in now. This is the spiritual battle which Christ our captain has already fought. But as we preach we must also point people forwards to the final battle, the dreadful and awful Day of the Lord when Yahweh will execute total judgement on the world and our only hope is to stand in the grace of God shown in Christ Jesus.
In other words, our warfare preaching, if I can call it that, must have both now and then aspects to it if we are to do justice to the sweep of the Bible's story.