Herem warfare is the commandment to exterminate all survivors. The law of herem warfare appears in the Book of Joshua and contravenes the usual motivations for going to war in the ancient Near East. Armies in that context typically killed only males and took women, children, and possessions before turning the conquered nation into a subjugated satellite nation. The law of herem warfare, on the other hand, forbade enslavement and required that all be killed.
Dru and Paul R. Hinlicky discuss this topic and more within the Book of Joshua. Dr. Hinlicky wrote Joshua for the Brazos Theological Commentary on the Bible series.
Dr. Hinlicky (PhD, Union Theological Seminary, 1983) is an internationally known theologian who has published more than seventy articles and many books. He teaches theology at Roanoke College in Salem, Virginia. He is an authority on the theology of Martin Luther and how Luther's theology played out in history since the time of the Reformation. 
Show notes:
00:26 Why don't we hear many sermons on the Book of Joshua?
02:55 The context of "be strong and courageous"
10:39 Herem warfare—does God command genocide?
15:17 The plan of herem warfare fails
25:32 The theological emphasis of the Book of Joshua

Show notes by Celina Durgin