The title of his fourth chapter, 'God will fight for Us', is an excellent indication of where Yoder is going in it. It's a brief attempt not to answer, but to reframe the question, of war and the Old Testament. Yoder points out that the question is generally frame as an ethical, generalised, and legalistic question, and then brought to the OT which has its own agenda.
Instead, Yoder points out that we need to read the scriptures as a story, and that the believing Israelite would not have read the story in that way. The strand of that story that Yoder teases out is God's salvation of his people, and he traces this through the Exodus event, the wars of conquest, the wars of the kingdom, right through to the post-exilic period. Yoder's point in all this is that the focus is on the deliverance of God, sometimes without violence, sometimes through violence, but often over and against politico-military strategy and reliance. His point is further strengthened by considering the way a lot of modern-ethically-problematic material (holy wars, Abraham/Isaac, etc) is framed cultically and non-problematically in the historico-social context. We impose questions that would have made no sense back then.
What Yoder does next is slightly unexpected. He's not here interested in elaborating a study of war and peace in the OT. Instead, he makes the point that with this kind of story, that believing Israelites in Jesus' day had an expectation that God could act in these kinds of ways. That Jesus' use of "the language of liberation and revolution, announcing a restoration of 'lingdom' community and a new pattern of life, without predicting or authorizing particular violent techniques for achieving his good ends, ... he need not have seemed to his listeners to be a dreamer." (p84)
Yoder then applies this leverage against the contemporary reader of Jesus. We assume that a generalised Jubilee, or a non-violent withdrawal of an enemy force, are highly improbably, if not downright impossible. So, we conclude Jesus could hardly have been talking of those kinds of things. But that is not the hearing Jesus would have received.
Likewise we treat apocalyptic sounding claims as "off the map", but the kinds of things Jesus is talking have a theological-historical grounding in the story of Israel in the Scriptures, so that "Jesus' proclamation of the kingdom was unacceptable to most of his listeners not because they thought it could not happen but because they feared it might, and that it would bring down judgment on them" (p85 Author's emphasis).
Personal thoughts: While it would have been great for Yoder to do far more work on the OT ethical material, that's not his aim here. And I should go and read some other texts for that myself. I do think Yoder's strategy is logically and persuasively powerful. We do approach the text with those kinds of wrong questions, and so fail to hear the texts on their own terms.
The same also applies to arguments I hear from Christians against my position. They practically, if not theoretically, think in terms of realpolitik, politico-military 'necessity', and success/failure. The idea of divine deliverance, of God acting in history, and of faithfulness and martyrdom being the marks of Christian 'success', are paid lip service and swept aside in ethical thinking.