Why does God command violence in the Old Testament?

This is one of the hardest questions in the Old Testament, and it deserves a careful, non-defensive answer.

Several things are important context: the conquest narratives use ancient Near Eastern hyperbolic war rhetoric common to the period (archaeology shows cities described as "totally destroyed" continued to be inhabited); the judgment came after centuries of patience and extreme moral corruption in Canaanite society, including child sacrifice (Leviticus 18:24-25); and God extended mercy to anyone who turned to Him, as with Rahab (Joshua 2, 6:25).

This remains a passage to wrestle with humbly rather than explain away entirely, but it should be read within the Bible's larger story of a God who is simultaneously just and, ultimately in Christ, the one who bears judgment Himself on behalf of sinners.